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
Change-Id: I645c013c39e3190fd96c4a549d39a331aced16bd