[PackageGraph] Ensure container is of the RepositoryContainer type

... when diagnosing unavailable versions.

<rdar://problem/51183216>

Author: aciidgh

Sources/PackageGraph/DependencyResolver.swift

@@ -316,6 +316,13 @@ public protocol PackageContainer {
     /// after the container is available. The updated identifier is returned in result of the
     /// dependency resolution.
     func getUpdatedIdentifier(at boundVersion: BoundVersion) throws -> PackageReference
+
+    /// Hack for the old resolver. Don't use.
+    var _isRemoteContainer: Bool? { get }
+}
+
+extension PackageContainer {
+    public var _isRemoteContainer: Bool? { return nil }
 }
 
 /// An interface for resolving package containers.
@@ -1006,6 +1013,8 @@ public class DependencyResolver {
         let constraintsWithNoAvailableVersions = constraints.filter { constraint in
             if case .versionSet(let versions) = constraint.requirement,
             let container = try? getContainer(for: constraint.identifier),
+            // FIXME: This is hacky but we should be moving away from this resolver anyway.
+            container._isRemoteContainer == true,
             !container.versions(filter: versions.contains).contains(where: { _ in true }) {
                 return true
             }

Sources/PackageGraph/RepositoryPackageContainerProvider.swift

@@ -324,6 +324,10 @@ public class RepositoryPackageContainer: BasePackageContainer, CustomStringConve
         return "RepositoryPackageContainer(\(identifier.repository.url.debugDescription))"
     }
 
+    public var _isRemoteContainer: Bool {
+        return true
+    }
+
     public func getTag(for version: Version) -> String? {
         return knownVersions[version]
     }

Sources/TestSupport/MockDependencyResolver.swift

@@ -149,6 +149,10 @@ public class MockPackageContainer: PackageContainer {
         self._versions = versions.sorted().reversed()
         self.dependencies = dependencies
     }
+
+    public var _isRemoteContainer: Bool? {
+        return true
+    }
 }
 
 public class MockPackageContainer2: MockPackageContainer {