Fix potential class initialization deadlocks

By ensuring that nested classes that extend the containing class, and are
referenced by the containing class' static initializer, cannot be initialized
from outside the containing class.

Without this change, deadlocks can occur if the nested and containing classes
are initialized by different threads, e.g.:

```
SpawnCache.NO_CACHE // thread 1
new ContainingPackageLookupValue.NoContainingPackage("") // thread 2
```

or:

```
new NoSpawnCache() // thread 1
ContainingPackageLookupValue.NONE // thread 2
```

PiperOrigin-RevId: 615919227
Change-Id: I3c513fa349c878cc604d2862a233e33a503c8293

src/main/java/com/google/devtools/build/lib/exec/SpawnCache.java

@@ -86,6 +86,8 @@ public static class NoSpawnCache implements SpawnCache {
     public CacheHandle lookup(Spawn spawn, SpawnExecutionContext context) {
       return SpawnCache.NO_RESULT_NO_STORE;
     }
+
+    private NoSpawnCache() {}
   }
 
   /** A no-op implementation that has no results and performs no stores. */

src/main/java/com/google/devtools/build/lib/skyframe/ContainingPackageLookupFunction.java

@@ -52,7 +52,7 @@ public SkyValue compute(SkyKey skyKey, Environment env) throws InterruptedExcept
     }
 
     if (ErrorReason.REPOSITORY_NOT_FOUND.equals(pkgLookupValue.getErrorReason())) {
-      return new ContainingPackageLookupValue.NoContainingPackage(pkgLookupValue.getErrorMsg());
+      return ContainingPackageLookupValue.noContainingPackage(pkgLookupValue.getErrorMsg());
     }
     PathFragment parentDir = dir.getPackageFragment().getParentDirectory();
     if (parentDir == null) {

src/main/java/com/google/devtools/build/lib/skyframe/ContainingPackageLookupValue.java

@@ -139,6 +139,10 @@ public static ContainingPackage withContainingPackage(PackageIdentifier pkgId, R
     return new ContainingPackage(pkgId, root);
   }
 
+  static ContainingPackageLookupValue noContainingPackage(String reason) {
+    return new NoContainingPackage(reason);
+  }
+
   /** Value indicating there is no containing package. */
   public static class NoContainingPackage extends ContainingPackageLookupValue {
     private final String reason;
@@ -147,7 +151,7 @@ private NoContainingPackage() {
       this.reason = null;
     }
 
-    NoContainingPackage(@Nonnull String reason) {
+    private NoContainingPackage(@Nonnull String reason) {
       this.reason = reason;
     }