Automatic merge from submit-queue (batch tested with PRs 40665, 41094, 41351, 41721, 41843)
parse single string as group rather than version
**What this PR does / why we need it**:
Prevent bad code generation by adjusting parsing of input.
When client-gen is run with `--input` as an unversioned type, it blows up if the group does not end with a trailing slash.
examples
1. `--input servicecatalog/v1alpha` is group servicecatalog, with version v1alpha1
1. `--input servicecatalog/` is group servicecatalog, with version empty string
1. `--input servicecatalog` is group empty string, with version servicecatalog
I disagree that the second and third cases are different. This adjusts them so they are both case 2.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*:
This change would address the concerns in PR https://github.com/kubernetes/community/pull/343
**Special notes for your reviewer**:
@caesarxuchao @pmorie this does not result in any change in any current code generation, and improves the user experience of client-gen. I ran `hack/update-codegen.sh` and saw no changes in git, nor did anything fail.
**Release note**:
```release-note
NONE
```
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