[6.0] Fix –-toolchain value shadowed by local executables (#7485)

Cherry-pick of #7483.

**Explanation**: Behavior of `--toolchain` regressed in https://github.com/apple/swift-package-manager/pull/7296/files by
preferring the first match in toolset root paths: when `swiftc` is available in the same directory next to the SwiftPM binary being
invoked, that became preferred with `--toolchain` value ignored.
**Scope**: limited to cross-compilation and infrequent builds that utilize this option.
**Risk**: low due to limited scope, merged to `main` for a week with no regressions discovered.
**Testing**: A newly added test verifies that the behavior is fixed. For that we had to inject `FileSystem` and `EnvironmentVariables` in a lot more places, with an added benefit of making our tests more consistent
and isolated from an arbitrary build environment.
**Issue**: rdar://126095653
**Reviewer**: @bnbarham

Author: MaxDesiatov

Sources/Basics/CMakeLists.txt

@@ -29,6 +29,7 @@ add_library(Basics
   Errors.swift
   FileSystem/AbsolutePath.swift
   FileSystem/FileSystem+Extensions.swift
+  FileSystem/InMemoryFileSystem.swift
   FileSystem/NativePathExtensions.swift
   FileSystem/RelativePath.swift
   FileSystem/TemporaryFile.swift

Sources/Basics/FileSystem/FileSystem+Extensions.swift

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift open source project
 //
-// Copyright (c) 2020-2021 Apple Inc. and the Swift project authors
+// Copyright (c) 2020-2024 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See http://swift.org/LICENSE.txt for license information
@@ -632,3 +632,48 @@ extension FileLock {
         return try Self.prepareLock(fileToLock: fileToLock.underlying, at: lockFilesDirectory?.underlying)
     }
 }
+
+/// Convenience initializers for testing purposes.
+extension InMemoryFileSystem {
+    /// Create a new file system with the given files, provided as a map from
+    /// file path to contents.
+    public convenience init(files: [String: ByteString]) {
+        self.init()
+
+        for (path, contents) in files {
+            let path = try! AbsolutePath(validating: path)
+            try! createDirectory(path.parentDirectory, recursive: true)
+            try! writeFileContents(path, bytes: contents)
+        }
+    }
+
+    /// Create a new file system with an empty file at each provided path.
+    public convenience init(emptyFiles files: String...) {
+        self.init(emptyFiles: files)
+    }
+
+    /// Create a new file system with an empty file at each provided path.
+    public convenience init(emptyFiles files: [String]) {
+        self.init()
+        self.createEmptyFiles(at: .root, files: files)
+    }
+}
+
+extension FileSystem {
+    public func createEmptyFiles(at root: AbsolutePath, files: String...) {
+        self.createEmptyFiles(at: root, files: files)
+    }
+
+    public func createEmptyFiles(at root: AbsolutePath, files: [String]) {
+        do {
+            try createDirectory(root, recursive: true)
+            for path in files {
+                let path = try AbsolutePath(validating: String(path.dropFirst()), relativeTo: root)
+                try createDirectory(path.parentDirectory, recursive: true)
+                try writeFileContents(path, bytes: "")
+            }
+        } catch {
+            fatalError("Failed to create empty files: \(error)")
+        }
+    }
+}