[Workspace] Add an option to force resolution

This will force the resolution even if the current checkout has
everything that is required. This can be used to "refresh" the managed
dependencies contents.

<rdar://problem/51463965>

Author: aciidgh

Sources/Workspace/Workspace.swift

@@ -489,7 +489,7 @@ extension Workspace {
                 container: dependency.packageRef, requirement: requirement)
 
         // Run the resolution.
-        _resolve(root: root, extraConstraints: [constraint], diagnostics: diagnostics)
+        _resolve(root: root, forceResolution: false, extraConstraints: [constraint], diagnostics: diagnostics)
     }
 
     /// Cleans the build artefacts from workspace data.
@@ -661,7 +661,7 @@ extension Workspace {
         if forceResolvedVersions {
             manifests = self._resolveToResolvedVersion(root: root, diagnostics: diagnostics)
         } else {
-            manifests = self._resolve(root: root, diagnostics: diagnostics)
+            manifests = self._resolve(root: root, forceResolution: false, diagnostics: diagnostics)
         }
         let externalManifests = manifests.allManifests()
 
@@ -697,9 +697,10 @@ extension Workspace {
     /// checkout will be restored according to its pin.
     public func resolve(
         root: PackageGraphRootInput,
+        forceResolution: Bool = false,
         diagnostics: DiagnosticsEngine
     ) {
-        _resolve(root: root, diagnostics: diagnostics)
+        _resolve(root: root, forceResolution: forceResolution, diagnostics: diagnostics)
     }
 
     /// Loads and returns manifests at the given paths.
@@ -1215,6 +1216,7 @@ extension Workspace {
     @discardableResult
     fileprivate func _resolve(
         root: PackageGraphRootInput,
+        forceResolution: Bool,
         extraConstraints: [RepositoryPackageConstraint] = [],
         diagnostics: DiagnosticsEngine,
         retryOnPackagePathMismatch: Bool = true
@@ -1265,7 +1267,7 @@ extension Workspace {
 
             // If we don't need resolution and there are no extra constraints,
             // just validate pinsStore and return.
-            if !result.resolve && extraConstraints.isEmpty {
+            if !result.resolve && extraConstraints.isEmpty && !forceResolution {
                 return currentManifests
             }
 
@@ -1345,6 +1347,7 @@ extension Workspace {
                 // we have the manifest files of all the dependencies.
                 return self._resolve(
                     root: root,
+                    forceResolution: forceResolution,
                     extraConstraints: extraConstraints,
                     diagnostics: diagnostics,
                     retryOnPackagePathMismatch: false