Due to this change, `./xbps-src sort-dependencies` will take
checkdepends into account when using -Q or -K.
Before this commit, if `pkgA` checkdepends on `pkgB`, sort-dependencies
could still print `pkgA` before `pkgB`. This causes CI to build `pkgB`
twice: first when building `pkgA`, which forces implicit build of pkgB;
second when building `pkgB` (explicit, so it will ignore the package is
already built).
The implementation uses `skip_check_step()` from previous commit, for
consistency, so checkdepends are only taken into account if the check
step would be enabled.
In particular, nothing is changed unless -Q or -K flag is passed.
EXAMPLE:
Before:
```
$ ./xbps-src -Q sort-dependencies python3-process-tests python3-pytest-cov
python3-pytest-cov
python3-process-tests
```
After:
```
$ ./xbps-src -Q sort-dependencies python3-process-tests python3-pytest-cov
python3-process-tests
python3-pytest-cov
```
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