pmb.parse.depends.package_provider: add type hinting and fix references (MR 2439)

pmb.parse.apkindex.providers now returns a dict of string:ApkindexBlock,
probably from 71772b9b

This adds type hinting to package_provider to reflect this, and fixes
code that calls it to properly deal with the possible return types.
Without this, building FDE images is broken.