More BUILD file refactorings.

Split collect, concurrent, vfs, windows into package-level BUILD files.
Move clock classes out of "util", into their own Java package.
Move CompactHashSet into its own Java package to break a dependency cycle.
Give nestedset and inmemoryfs their own package-level BUILD files.

PiperOrigin-RevId: 167702127
diff --git a/src/test/java/com/google/devtools/build/lib/skyframe/TimestampBuilderTestCase.java b/src/test/java/com/google/devtools/build/lib/skyframe/TimestampBuilderTestCase.java
index 0357d70..1143d3c 100644
--- a/src/test/java/com/google/devtools/build/lib/skyframe/TimestampBuilderTestCase.java
+++ b/src/test/java/com/google/devtools/build/lib/skyframe/TimestampBuilderTestCase.java
@@ -50,6 +50,8 @@
 import com.google.devtools.build.lib.analysis.ConfiguredTarget;
 import com.google.devtools.build.lib.analysis.TopLevelArtifactContext;
 import com.google.devtools.build.lib.buildtool.SkyframeBuilder;
+import com.google.devtools.build.lib.clock.BlazeClock;
+import com.google.devtools.build.lib.clock.Clock;
 import com.google.devtools.build.lib.events.Reporter;
 import com.google.devtools.build.lib.events.StoredEventHandler;
 import com.google.devtools.build.lib.exec.SingleBuildFileCache;
@@ -64,8 +66,6 @@
 import com.google.devtools.build.lib.testutil.TestRuleClassProvider;
 import com.google.devtools.build.lib.testutil.TestUtils;
 import com.google.devtools.build.lib.util.AbruptExitException;
-import com.google.devtools.build.lib.util.BlazeClock;
-import com.google.devtools.build.lib.util.Clock;
 import com.google.devtools.build.lib.util.Preconditions;
 import com.google.devtools.build.lib.util.io.TimestampGranularityMonitor;
 import com.google.devtools.build.lib.vfs.FileSystem;