Rename bzl files to scl

These files don't have anything to do with bazel, they just use starlark as a configuration
language. Bazel recently introduced the scl file extension to use for this format, which doesn't
have any bazel-specific symbols. Use that extension for our pure starlark files as well.

Bug: 309686282
Test: Presubmits
Change-Id: I04719332ca781f98a459b0038df37029efcbd8b1