Use async HTTPClient in BinaryArtifactsManager (#8087)

### Motivation:

To adopt Swift concurrency in `ManifestLoader`, we need to convert
`RegistryClient` first, which then would require use of `HTTPClient`
instead of `LegacyHTTPClient`. That would also cascade to
`BinaryArtifactsManager`, so I'm starting there first to make PRs
smaller.

### Modifications:

Converted (and as a consequence, simplified) code in
`Workspace+BinaryArtifacts.swift`, also adopted `HTTPClient` in
`WorkspaceTests` to fix tests after the change.

### Result:

One major previous adopter of `LegacyHTTPClient` is now compatible with
Swift concurrency.

Since `BinaryArtifactsManager` was blocking Swift concurrency thread
pool and also used `.sharedConcurrent` dispatch queue, this potentially
fixes thread explosion/deadlocks that may have been caused by previously
broken code.

Author: MaxDesiatov

Sources/Basics/Concurrency/ThreadSafeBox.swift

@@ -30,6 +30,12 @@ public final class ThreadSafeBox<Value> {
             self.underlying = value
         }
     }
+    
+    public func mutate(body: (inout Value?) throws -> ()) rethrows {
+        try self.lock.withLock {
+            try body(&self.underlying)
+        }
+    }
 
     @discardableResult
     public func memoize(body: () throws -> Value) rethrows -> Value {

Sources/Workspace/Diagnostics.swift

@@ -123,59 +123,67 @@ extension Basics.Diagnostic {
     static func customDependencyMissing(packageName: String) -> Self {
         .warning("dependency '\(packageName)' is missing; retrieving again")
     }
+}
+
+struct BinaryArtifactsManagerError: Error, CustomStringConvertible {
+    let description: String
+
+    private init(description: String) {
+        self.description = description
+    }
 
     static func artifactInvalidArchive(artifactURL: URL, targetName: String) -> Self {
-        .error(
-            "invalid archive returned from '\(artifactURL.absoluteString)' which is required by binary target '\(targetName)'"
+        .init(
+            description: "invalid archive returned from '\(artifactURL.absoluteString)' which is required by binary target '\(targetName)'"
         )
     }
 
     static func artifactChecksumChanged(targetName: String) -> Self {
-        .error(
-            "artifact of binary target '\(targetName)' has changed checksum; this is a potential security risk so the new artifact won't be downloaded"
+        .init(
+            description: "artifact of binary target '\(targetName)' has changed checksum; this is a potential security risk so the new artifact won't be downloaded"
         )
     }
 
     static func artifactInvalidChecksum(targetName: String, expectedChecksum: String, actualChecksum: String?) -> Self {
-        .error(
-            "checksum of downloaded artifact of binary target '\(targetName)' (\(actualChecksum ?? "none")) does not match checksum specified by the manifest (\(expectedChecksum))"
+        .init(
+            description: "checksum of downloaded artifact of binary target '\(targetName)' (\(actualChecksum ?? "none")) does not match checksum specified by the manifest (\(expectedChecksum))"
         )
     }
 
     static func artifactFailedDownload(artifactURL: URL, targetName: String, reason: String) -> Self {
-        .error(
-            "failed downloading '\(artifactURL.absoluteString)' which is required by binary target '\(targetName)': \(reason)"
+        .init(
+            description: "failed downloading '\(artifactURL.absoluteString)' which is required by binary target '\(targetName)': \(reason)"
         )
     }
 
     static func artifactFailedValidation(artifactURL: URL, targetName: String, reason: String) -> Self {
-        .error(
-            "failed validating archive from '\(artifactURL.absoluteString)' which is required by binary target '\(targetName)': \(reason)"
+        .init(
+            description: "failed validating archive from '\(artifactURL.absoluteString)' which is required by binary target '\(targetName)': \(reason)"
         )
     }
 
     static func remoteArtifactFailedExtraction(artifactURL: URL, targetName: String, reason: String) -> Self {
-        .error(
-            "failed extracting '\(artifactURL.absoluteString)' which is required by binary target '\(targetName)': \(reason)"
+        .init(
+            description: "failed extracting '\(artifactURL.absoluteString)' which is required by binary target '\(targetName)': \(reason)"
         )
     }
 
     static func localArtifactFailedExtraction(artifactPath: AbsolutePath, targetName: String, reason: String) -> Self {
-        .error("failed extracting '\(artifactPath)' which is required by binary target '\(targetName)': \(reason)")
+        .init(description: "failed extracting '\(artifactPath)' which is required by binary target '\(targetName)': \(reason)")
     }
 
     static func remoteArtifactNotFound(artifactURL: URL, targetName: String) -> Self {
-        .error(
-            "downloaded archive of binary target '\(targetName)' from '\(artifactURL.absoluteString)' does not contain a binary artifact."
+        .init(
+            description: "downloaded archive of binary target '\(targetName)' from '\(artifactURL.absoluteString)' does not contain a binary artifact."
         )
     }
 
     static func localArchivedArtifactNotFound(archivePath: AbsolutePath, targetName: String) -> Self {
-        .error("local archive of binary target '\(targetName)' at '\(archivePath)' does not contain a binary artifact.")
+        .init(description: "local archive of binary target '\(targetName)' at '\(archivePath)' does not contain a binary artifact.")
     }
 
     static func localArtifactNotFound(artifactPath: AbsolutePath, targetName: String) -> Self {
-        .error("local binary target '\(targetName)' at '\(artifactPath)' does not contain a binary artifact.")
+        .init(description: "local binary target '\(targetName)' at '\(artifactPath)' does not contain a binary artifact.")
     }
 
     static func exhaustedAttempts(missing: [PackageReference]) -> Self {
@@ -189,8 +197,8 @@ extension Basics.Diagnostic {
                 return "'\($0.identity)' at \(path)"
             }
         }
-        return .error(
-            "exhausted attempts to resolve the dependencies graph, with the following dependencies unresolved:\n* \(missing.joined(separator: "\n* "))"
+        return .init(
+            description: "exhausted attempts to resolve the dependencies graph, with the following dependencies unresolved:\n* \(missing.joined(separator: "\n* "))"
         )
     }
 }