Ensure we don't have any broken links to external repository labels

bazelbuild@f27cfb8 made it possible to generate docs for rules from other repos but unless `src/main/java/com/google/devtools/build/docgen/bazel_link_map.json` is also updated appropriately, we can end up with broken links to rule sources.

PiperOrigin-RevId: 704725413
Change-Id: I9eb5c490a1bfe9aa1c4325d394bc4840dc574aab

src/main/java/com/google/devtools/build/docgen/SourceUrlMapper.java

@@ -95,11 +95,9 @@ private String urlOfLabel(Label label) {
             + label.toPathFragment().getPathString();
     for (Map.Entry<String, String> entry : repoPathsRewrites.entrySet()) {
       if (path.startsWith(entry.getKey())) {
-        path = entry.getValue() + path.substring(entry.getKey().length());
-        break;
+        return entry.getValue() + path.substring(entry.getKey().length());
       }
     }
-
-    return path;
+    throw new IllegalStateException("Label URL left untransformed: " + path);
   }
 }

src/test/java/com/google/devtools/build/docgen/BuildDocCollectorTest.java

@@ -48,7 +48,9 @@ public final class BuildDocCollectorTest {
       new SourceUrlMapper(
           /* sourceUrlRoot= */ "https://example.com/",
           /* inputRoot= */ "/tmp/io_bazel/",
-          ImmutableMap.of("@_builtins//", "https://example.com/src/main/starlark/builtins_bzl/"));
+          ImmutableMap.of(
+              "@//", "https://main-repo.com/",
+              "@_builtins//", "https://example.com/src/main/starlark/builtins_bzl/"));
 
   @Before
   public void setUpCollectorState() {