Follow up to #1162.
* `pmb.build.buildinfo()`: Used to record the build environment. It is
flawed because it scans the repo APKINDEX files instead of using the
actually installed packages list. When it was implemented we were not
able to do the latter. After this is removed, `pmb.parse.depends` can
be simplified (it needs to be rewritten for #1122).
* `pmb.helpers.repo.diff()` and `pmb.helpers.repo.files()`: These were
used exclusively by `pmb.build.buildinfo()`, to learn about which
files have been changed in the local repository folder after a
package was built. The idea was, that we could find subpackages that
way. But this information is present in the installed package list as
well, which is a much cleaner approach.
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