appstream extractor should search valid directories

Currently the appstream extractor requires `parse-info` to contain the complete path for appstream files. This is excessively repetitive and useless information as there are only two well defined directories where the files may be located. Should that change in the future it also means every developer needs to update the paths instead of snapcraft doing it once for everyone by updating the search paths there.

It would be far superior if the developer specified `org.example.foo.appdata.xml` and the extractor looked for that file in $stage/usr/share/{appinfo,metainfo}/ and relative to snapcraft PWD.