Automatic merge from submit-queue (batch tested with PRs 55841, 55948, 55945). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

conversion-gen: check for nil pkg in getManualConversionFunctions

**What this PR does / why we need it**:

Following https://github.com/kubernetes/kubernetes/pull/54394, this PR performs a check in getManualConversionFunctions for packages that are not found and prints a warning instead of panicking.

This should improve the UX of conversion-gen, and is also a narrower change than that proposed in #54394, so hopefully can be cherry picked into the release-1.8 etc. branches.

This would allow users to use conversion-gen via the generate-internal-groups.sh script without having to make a second call that 'blanks out' `--extra-peer-dirs`.

/cc @sttts

```release-note
NONE
```