[PackageGraph] Actually implement _isRemoteContainer

<rdar://problem/51664794>

Author: aciidgh

Sources/PackageGraph/DependencyResolver.swift

@@ -1010,6 +1010,7 @@ public class DependencyResolver {
 
     /// Diagnoses missing versions for the given constraints.
     private func diagnoseMissingVersions(for constraints: [RepositoryPackageConstraint]) throws {
+
         let constraintsWithNoAvailableVersions = constraints.filter { constraint in
             if case .versionSet(let versions) = constraint.requirement,
             let container = try? getContainer(for: constraint.identifier),

Sources/PackageGraph/RepositoryPackageContainerProvider.swift

@@ -164,6 +164,10 @@ public class BasePackageContainer: PackageContainer {
         self.toolsVersionLoader = toolsVersionLoader
         self.currentToolsVersion = currentToolsVersion
     }
+
+    public var _isRemoteContainer: Bool? {
+        return nil
+    }
 }
 
 /// Local package container.
@@ -324,7 +328,7 @@ public class RepositoryPackageContainer: BasePackageContainer, CustomStringConve
         return "RepositoryPackageContainer(\(identifier.repository.url.debugDescription))"
     }
 
-    public var _isRemoteContainer: Bool {
+    public override var _isRemoteContainer: Bool? {
         return true
     }