Move find_aport() and find_aport_guess_main() from pmb/build/other.py
to the new file pmb/helpers/pmaports.py.
Finding aports is not only needed when building packages, hence it
makes sense to move it out of pmb.build. The pmb/helpers/pmaports.py
file will have more pmaports related functions in a follow up commit.
* Allow to specify a custom username in "pmbootstrap init"
* Build chroots have "pmos" instead of "user" as username now
* Installation user UID is 1000 now (as in all other Linux distributions)
* Adjust autologins
* postmarketos-base: enable wheel group for sudo, removed previous sudoers file
* Implement safe upgrade path:
We save the version of the work folder format now, in $WORK/version.
When this file does not exist, it defaults to 0.
In case it does not match the currently required version
(pmb.config.work_version), then ask the user if it should
automatically be upgraded.