Buildozer - allow overriding build file names

There's a few different use cases where it would be useful to specify custom file names which are identified and processed by Buildozer.
This could for instance be enabled via a command line flag.

Note: File names are referenced in a number of places to for instance determine whether a file should be processed as `.bzl` or a `BUILD` file, so this change would need to be done holistically, making sure that all the behaviors which depend on the file name are supported, and that overriding the default file names does not produce unexpected side effects.

(Based to a large extent on PR #168)

Start by tracing Buildozer's file-name detection and the command-line flag entry points. Identify every behavior that distinguishes .bzl and BUILD files, then verify that custom names are handled consistently without unexpected side effects.

Written by the indexing model from the issue text.