Add -p, -e options in merge_zips for supporting par format.

Move __init__.py files population to merge_zips, and add options: -p, and
-e. Since par needs entry_point.txt to search the program entry point, so
add -e option to support it.

Test: ./merge_zips -p -e entry_point.txt out in1.zip in2.zip
Bug: 70568913
Change-Id: Ida10faf125e1b94dffc98b30240db3b90fd75b25