Revert "Don't assume all files called ".a" are ELF binaries. This prevents an...

Revert "Don't assume all files called ".a" are ELF binaries. This prevents an "Unrecognized archive format" traceback when processing (for example) lie 2.2.2+dfsg-3."

This reverts commit a1b57f81 as it causes
regressions in the rlib tests:

tests/comparators/test_rlib.py::test_identification FAILED               [ 81%]
tests/comparators/test_rlib.py::test_no_differences PASSED               [ 82%]
tests/comparators/test_rlib.py::test_num_items FAILED                    [ 82%]
tests/comparators/test_rlib.py::test_item0_armap FAILED                  [ 82%]
tests/comparators/test_rlib.py::test_item1_elf FAILED                    [ 82%]
tests/comparators/test_rlib.py::test_item2_rust_metadata_bin FAILED      [ 83%]
tests/comparators/test_rlib.py::test_item3_deflate_llvm_bitcode FAILED   [ 83%]
tests/comparators/test_rlib.py::test_compare_non_existing FAILED         [ 83%]