add3prf.py: Support license files with extensions

Some crates place there license files in files with extensions, such as
LICENSE.txt or LICENSE-MIT.md.

We should account for extensions by removing them.

Bug: 321064939
Test: Ran script against `minimal-lexical-0.2.1` crate and added unit
      tests
Change-Id: I0c385e8209969c0900909e764460fc551f07c20c
2 files changed