Kotlin: address review comments

java/kotlin-extractor/BUILD.bazel

@@ -2,12 +2,11 @@
 # Usage overview
 Building the extractor can be done via
 ```
-bazel build //java/kotlin-extractor:codeql-extractor-kotlin-<variant>-<version>
+bazel build @codeql//java/kotlin-extractor:codeql-extractor-kotlin-<variant>-<version>
 ```
 where `<variant>` is either `standalone` or `embeddable`, and `<version>` is one of the supported versions.
-
 ```
-bazel build //java/kotlin-extractor
+bazel build @codeql//java/kotlin-extractor
 ```
 will build a default variant:
 * standalone, unless `CODEQL_KOTLIN_SINGLE_VERSION_EMBEDDABLE` is set to true, in which case it will go for embeddable
@@ -19,6 +18,8 @@ If `kotlinc` is updated, bazel won't be aware of it and will therefore keep the
 * `bazel clean`
 * `bazel fetch --force @codeql_kotlin_defaults\\:all`
 * `CODEQL_KOTLIN_SINGLE_VERSION= bazel build //java/kotlin-extractor`
+
+If building from the `codeql` repository, `@codeql` can be skipped.
 """
 
 # This file is used in the `@codeql_kotlin_embeddable` external repo, which means we need to

java/kotlin-extractor/deps.bzl

@@ -49,8 +49,8 @@ def _walk(dir):
             next_dirs.extend([c for c in children if c.is_dir])
             res.extend([c for c in children if not c.is_dir])
         if not next_dirs:
-            break
-    return res
+            return res
+    fail("%s directory too deep" % dir)
 
 def _embeddable_source_impl(repository_ctx):
     src_dir = repository_ctx.path(Label("//java/kotlin-extractor:src"))