Add presubmit check to //mojo/{public, edk} for BUILD.gn file flexibility.

This script checks for changes to the build files that would break the ability
to drop the mojo SDK EDK into a client repo at a location other than
//mojo/{public, edk} and have GN work:

- References to "//mojo/public" in the SDK (these should be relative paths).
- References to other absolute paths in the SDK (these shouldn't be there at
  all).
- References to "//mojo/public" and "//mojo/edk" in the EDK (these should be
  relative paths).
- Source set targets that are not constructed via the appropriate wrapper
  (mojo_sdk_source_set or mojo_edk_source_set targets as appropriate).

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/782743002