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.

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MOS_MIGRATED_REVID=109676408
diff --git a/src/test/java/com/google/devtools/build/lib/skyframe/PackageLookupFunctionTest.java b/src/test/java/com/google/devtools/build/lib/skyframe/PackageLookupFunctionTest.java
index 6b540e5..a27cb61 100644
--- a/src/test/java/com/google/devtools/build/lib/skyframe/PackageLookupFunctionTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skyframe/PackageLookupFunctionTest.java
@@ -73,7 +73,7 @@
     AtomicReference<PathPackageLocator> pkgLocator = new AtomicReference<>(
         new PathPackageLocator(outputBase, ImmutableList.of(emptyPackagePath, rootDirectory)));
     deletedPackages = new AtomicReference<>(ImmutableSet.<PackageIdentifier>of());
-    ExternalFilesHelper externalFilesHelper = new ExternalFilesHelper(pkgLocator);
+    ExternalFilesHelper externalFilesHelper = new ExternalFilesHelper(pkgLocator, false);
     TimestampGranularityMonitor tsgm = new TimestampGranularityMonitor(BlazeClock.instance());
     BlazeDirectories directories = new BlazeDirectories(rootDirectory, outputBase, rootDirectory);
 
@@ -84,7 +84,7 @@
         SkyFunctions.PACKAGE,
         new PackageFunction(null, null, null, null, null, null, null));
     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));
     skyFunctions.put(SkyFunctions.BLACKLISTED_PACKAGE_PREFIXES,
         new BlacklistedPackagePrefixesFunction());
     RuleClassProvider ruleClassProvider = TestRuleClassProvider.getRuleClassProvider();