Add back inject_init rule

[Eric-Arellano]

#9115 inlined inject_init.py into prepare_chrooted_python_sources.py because it was the only call site at the time.

Turns out, V2 coverage has a use case for inject_init.

This makes several enhancements over the original inject_init implementation:

• Uses InputFilesContent instead of ExecuteProcessRequest
• The result merges the original with new __init__.pys, rather than returning only the newly added __init__.pys.
• Returns a Snapshot, rather than Digest, which is more ergonomic.
• More comprehensive tests, e.g. ensuring we aren't overriding the original __init__.py.

[TansyArron]

Thanks Eric!