appstream extractor should search valid directories

[Harald Sitter]

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.

[Sergio Schvezov (sergiusens)]

sergiusens: git clone --branch Neon/unstable https://anongit.neon.kde.org/kde/kblocks.git
12:21 sergiusens: depending on what you want to do kbruch.git might be better though as it has no additional deps besides the build-snap. in either case you'll need `export SNAPCRAFT_PARTS_URI=https://metadata.neon.kde.org/snap/parts.yaml` for the remaining remote part