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/main/java/com/google/devtools/build/lib/query2/SkyQueryEnvironment.java b/src/main/java/com/google/devtools/build/lib/query2/SkyQueryEnvironment.java
index 5229575..1374652 100644
--- a/src/main/java/com/google/devtools/build/lib/query2/SkyQueryEnvironment.java
+++ b/src/main/java/com/google/devtools/build/lib/query2/SkyQueryEnvironment.java
@@ -41,7 +41,7 @@
 import com.google.devtools.build.lib.cmdline.PackageIdentifier;
 import com.google.devtools.build.lib.cmdline.TargetParsingException;
 import com.google.devtools.build.lib.cmdline.TargetPattern;
-import com.google.devtools.build.lib.collect.CompactHashSet;
+import com.google.devtools.build.lib.collect.compacthashset.CompactHashSet;
 import com.google.devtools.build.lib.concurrent.BlockingStack;
 import com.google.devtools.build.lib.concurrent.MultisetSemaphore;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;