[PubGrub] Fix a potential deadlock when prefetching

The prefetching logic had a bug which sometimes leads to deadlock if
prefetched container queries start calling the completion block while
the main resolution logic is trying to fetch a new container that is NOT
in the prefetching list (because it's new dependency). The main reason
for this is that the container provider is using a serial queue for
callbacks (which is questionable) but it's better to not make any
assumptions about the provider and never run potentially blocking code
in the compleition handler.

Unfortunately, this is proving hard to unit test because this
depends on network speed/latency etc.

<rdar://problem/58644686>

Sources/PackageGraph/Pubgrub.swift

@@ -11,6 +11,7 @@
 import struct TSCUtility.Version
 import TSCBasic
 import struct PackageModel.PackageReference
+import Dispatch
 
 /// A term represents a statement about a package that may be true or false.
 public struct Term: Equatable, Hashable {
@@ -1845,15 +1846,17 @@ private final class ContainerProvider {
                 _prefetchingContainers.insert(identifier)
 
                 provider.getContainer(for: identifier, skipUpdate: skipUpdate) { container in
-                    self.fetchCondition.whileLocked {
-                        // Update the structures and signal any thread waiting
-                        // on prefetching to finish.
-                        let pubGrubContainer = container.map {
-                            PubGrubPackageContainer($0, pinsMap: self.pinsMap)
+                    DispatchQueue.global().async {
+                        self.fetchCondition.whileLocked {
+                            // Update the structures and signal any thread waiting
+                            // on prefetching to finish.
+                            let pubGrubContainer = container.map {
+                                PubGrubPackageContainer($0, pinsMap: self.pinsMap)
+                            }
+                            self._fetchedContainers[identifier] = pubGrubContainer
+                            self._prefetchingContainers.remove(identifier)
+                            self.fetchCondition.signal()
                         }
-                        self._fetchedContainers[identifier] = pubGrubContainer
-                        self._prefetchingContainers.remove(identifier)
-                        self.fetchCondition.signal()
                     }
                 }
             }