Support merging target files from directory

Expand `merge_target_files.py` API capabilities so that
`--framework-target-files` and `--vendor-target-files`
can be either zip archives or directories.

Test: Create a merged package by vendor target files folder
Test: atest --host releasetools_test
Bug: 276068400
Change-Id: I200be2a458ae59a61e05bfd7c78ab66093db32eb