bzl_library "builds" with missing deps

As seen in #49, you can update a .bzl file to add another load statement, but failing to add the dep for the `skylark_library` doesn't cause the build to break. If the `skylark_library` is then used to copy files for something like integration tests, those will fail as the file won't be found in the integration test's sandbox to be `load()ed`.

Start by reading the behavior described in #49 and locating the skylark_library rule and its handling of load statements and declared dependencies. Reproduce the missing-dependency case, including use from an integration test sandbox; done means the build fails or otherwise reports the absent dependency before the copied file is loaded.

Written by the indexing model from the issue text.