Only allow .scl files to load other .scl files

.scl files are starlark configuraiont language files, which is starlark
without any of the bazel buildin symbols.

This is to match bazel's behavior, and it doesn't make sense for
scl files to load bzl files.

We should change all the release config bzl files to scl, and then
also make it so that when using executionModeScl you can only load
scl files.

Test: go test
Change-Id: I196bcf3a4548118791ed1d47c2d37e46a1ef86c4