update_crate_tests: Clean up

Rename the Crate class to Package as this script may also be used to
update TEST_MAPPING for the prebuilts/rust toolchain.

Inline get_tests function as one branch was never used.

Rename `create_test_mapping` to `create` as it is already a method of

Test: update_crate_tests.py in external/rust/crates/libc
Test: update_crate_tests.py in prebuilts/rust
Change-Id: I06bd6d4f93223bb50fdb898980bd25fa3cea9019
1 file changed