pw_presubmit: Search BUILD.gn files directly

- Support directly searching BUILD.gn files for path names. This finds
  files more reliably than gn desc in some circumstances.
- List a few sources in source_sets in BUILD.gn files instead of
  referring directly to these files from elsewhere. This makes it
  possible to find these files using the new BUILD.gn searching method.
- Switch to directly searching BUILD.gn files rather than using gn desc.
  This approach is faster and ensures that sources are listed in their
  BUILD.gn files rather than directly referred to from elsewhere.

Change-Id: Ife525143d3d70f6a19719027e02bda21263f12eb
diff --git a/pw_fuzzer/BUILD.gn b/pw_fuzzer/BUILD.gn
index 6b67150..61271c3 100644
--- a/pw_fuzzer/BUILD.gn
+++ b/pw_fuzzer/BUILD.gn
@@ -77,6 +77,15 @@
   public_deps = [ "$dir_pw_log" ]
 }
 
+source_set("run_as_unit_test") {
+  configs = [ ":default_config" ]
+  sources = [ "pw_fuzzer_disabled.cc" ]
+  deps = [
+    dir_pw_log,
+    dir_pw_unit_test,
+  ]
+}
+
 # See https://llvm.org/docs/LibFuzzer.html#fuzzer-friendly-build-mode
 config("fuzzing_build_mode_unsafe_for_production") {
   defines = [ "FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION" ]