Preserve *.bp.fragment files

This is actually better than using *.bp as a suffix because then bpfmt
doesn't try to format them and fail.

It also fixes problems with migrating the nix crate.

Bug: http://b/339424309
Test: treehugger
Change-Id: Iee613e2416d8ca2a8b0b264c5bd2a5abfcae928b
1 file changed