[tint] Improvements to './tools/run gen build'

Add support for internal and external AdditionalDependencies in BUILD.cfg.

Move gtest to 'externals.json'

Document 'externals.json'

Change-Id: I143210c27ef85db6ce06b427ea75a1feb30e5df4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/147321
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/src/tint/lang/wgsl/sem/BUILD.cmake b/src/tint/lang/wgsl/sem/BUILD.cmake
index 3503a14..6d27566 100644
--- a/src/tint/lang/wgsl/sem/BUILD.cmake
+++ b/src/tint/lang/wgsl/sem/BUILD.cmake
@@ -140,3 +140,7 @@
   "utils/text"
   "utils/traits"
 )
+
+tint_target_add_external_dependencies("lang/wgsl/sem:test"
+  "gtest"
+)