This is a primitive check to ensure that .bp, .gn and .bazel files don't have source references to missing files.

This can catch a common build breakage.

Fixes: b/153439736
Change-Id: I95621f8775a0e536015d4da9897785935f130884
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43572
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>