pmb.parse.depends.package_provider: add type hinting and fix references

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.

Important Without this patch, building FDE images is broken.

cc @Newbyte