pmbootstrap doesn't support explicitly conflicting depdencies (depends="!foo")

APKBUILD supports marking another package as explicitly conflicting with the current package via

depends="!conflicting-package ..."

pmbootstrap currently doesn't support this and tries to find a package named "!conflicting-package" instead.

$ cat main/unl0kr/APKBUILD
...
depends="
        ...
        !osk-sdl
        "
...
$ pmbootstrap build --force unl0kr
[20:26:06] ERROR: Package '!osk-sdl': Could not find aport, and could not find this package in any APKINDEX!
[20:26:06] See also: <https://postmarketos.org/troubleshooting>
Run 'pmbootstrap log' for details.