Don't treat external files as immutable

Fixes #352.

RELNOTES: Files in external repositories are now treated as mutable, which will make the correctness guarantees of using external repositories stronger (existent), but may cause performance penalties.

--
MOS_MIGRATED_REVID=109676408
diff --git a/src/test/java/com/google/devtools/build/lib/skyframe/ContainingPackageLookupFunctionTest.java b/src/test/java/com/google/devtools/build/lib/skyframe/ContainingPackageLookupFunctionTest.java
index 83c8775..3d7f77e 100644
--- a/src/test/java/com/google/devtools/build/lib/skyframe/ContainingPackageLookupFunctionTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skyframe/ContainingPackageLookupFunctionTest.java
@@ -60,7 +60,7 @@
     AtomicReference<PathPackageLocator> pkgLocator =
         new AtomicReference<>(new PathPackageLocator(outputBase, ImmutableList.of(rootDirectory)));
     deletedPackages = new AtomicReference<>(ImmutableSet.<PackageIdentifier>of());
-    ExternalFilesHelper externalFilesHelper = new ExternalFilesHelper(pkgLocator);
+    ExternalFilesHelper externalFilesHelper = new ExternalFilesHelper(pkgLocator, false);
     TimestampGranularityMonitor tsgm = new TimestampGranularityMonitor(BlazeClock.instance());
 
     Map<SkyFunctionName, SkyFunction> skyFunctions = new HashMap<>();
@@ -69,7 +69,7 @@
     skyFunctions.put(SkyFunctions.BLACKLISTED_PACKAGE_PREFIXES,
         new BlacklistedPackagePrefixesFunction());
     skyFunctions.put(SkyFunctions.FILE_STATE, new FileStateFunction(tsgm, externalFilesHelper));
-    skyFunctions.put(SkyFunctions.FILE, new FileFunction(pkgLocator, tsgm, externalFilesHelper));
+    skyFunctions.put(SkyFunctions.FILE, new FileFunction(pkgLocator));
     RecordingDifferencer differencer = new RecordingDifferencer();
     evaluator = new InMemoryMemoizingEvaluator(skyFunctions, differencer);
     driver = new SequentialBuildDriver(evaluator);