Change bp2build converter of module "package".

1) Use attribute name "default_package_metadata" instead of
   "default_applicable_licenses" in packages to better describe its use.
2) Add a filegroup "default_metadata_file" in packages to search for
   METADATA file in each package.
3) Include "default_metadata_file" in each package's
   "default_package_metadata" attribute.

Bug: 275472038
Test: CIs
(cherry picked from https://android-review.googlesource.com/q/commit:2c9e8d612819840020b2d110d23fb2771b6c92f0)
Merged-In: I645c013c39e3190fd96c4a549d39a331aced16bd
Change-Id: I645c013c39e3190fd96c4a549d39a331aced16bd