Better heuristic to find license types

* Handle lower case file names like LICENSE-Apache.
* Use "license" key value in Cargo.toml if LICENSE-* files are missing.
* Recognize license type: ISC
* Improve readability of a sequence of if-statements.

Bug: 170254513
Test: get_rust_pkg.py --add3prf time-macros-impl
Test: run add3prf.py in current rust/crates/* projects
Change-Id: I9642f52090b20922302513a94fa97d41277f9762
1 file changed