Intern repository mapping entries

All repositories generated by a module extension have the same repository mapping entries. Without interning, if a module extension generates N repositories, each such repository would have its own copy with (more than) N entries, resulting in overall memory usage quadratic in N.

We intern the entries, not the `RepositoryMapping` itself, since the latter also includes the owner repo, which differs between extension repos.

Output of `bazel info used-heap-size-after-gc` after running a build with a synthetic module extension generating N + 1 repos and requesting all of them:

```
before
N=100:     32MB
N=1000:    77MB
N=3000:   371MB
N=5000:   961MB
N=10000: 3614MB

after
N=100:     32MB
N=1000:    44MB
N=3000:    71MB
N=5000:    91MB
N=10000:  158MB
```

Closes #19269.

PiperOrigin-RevId: 558940840
Change-Id: I07402f203b5f11bf448a1ae9e9ee4637ad4c536d

src/main/java/com/google/devtools/build/lib/cmdline/RepositoryMapping.java

@@ -17,6 +17,8 @@
 import com.google.auto.value.AutoValue;
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.Interner;
+import com.google.devtools.build.lib.concurrent.BlazeInterners;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -34,6 +36,15 @@
 @AutoValue
 public abstract class RepositoryMapping {
 
+  // All repositories generated by a module extension have the same repository mapping
+  // entries. Without interning, if a module extension generates N repositories, each such
+  // repository would have its own copy with (more than) N entries, resulting in memory usage that
+  // is quadratic in N.
+  // Note: We intern the entries, not the RepositoryMapping itself, because the latter also includes
+  // the owner repo, which differs between extension repos.
+  private static final Interner<ImmutableMap<String, RepositoryName>> ENTRIES_INTERNER =
+      BlazeInterners.newWeakInterner();
+
   // Always fallback to the requested name
   public static final RepositoryMapping ALWAYS_FALLBACK = createAllowingFallback(ImmutableMap.of());
 
@@ -60,7 +71,8 @@ public static RepositoryMapping createAllowingFallback(Map<String, RepositoryNam
 
   private static RepositoryMapping createInternal(
       Map<String, RepositoryName> entries, RepositoryName ownerRepo) {
-    return new AutoValue_RepositoryMapping(ImmutableMap.copyOf(entries), ownerRepo);
+    return new AutoValue_RepositoryMapping(
+        ENTRIES_INTERNER.intern(ImmutableMap.copyOf(entries)), ownerRepo);
   }
 
   /**