Better /usr merge support

The existing apk-tools patches for /usr merge have a few edgecase bugs, and overall take fully the wrong approach (rewriting paths at the point the files are installed rather than at the database level, breaking apk info and apk audit as well as leading to issues like https://gitlab.com/postmarketOS/pmbootstrap/-/issues/2388).

Upstream would prefer to implement this /usr merge support on apk3 and backport it to apk2, we may consider simply switching to apk3.

I have rewritten the /usr merge patch on apk3: https://gitlab.alpinelinux.org/alpine/apk-tools/-/merge_requests/155

And backported it to apk2: https://gitlab.com/postmarketOS/apk-tools/-/tree/caleb/apk2-usrmerge-db?ref_type=heads

We now need to:

• Update our fork of apk-tools with the backported new patches
• Test extensively (maybe ask people for their /etc/apk/world and check in VM?)
• 🚢 it