Empower vendir to make an informed decision on what the overall semver of a directory is

**Describe the problem/challenge you have**
After I sync things with vendir and create my informed view of a folder I need to make a decision later on about what overall semantically appropriate version this folder is. Today I do that with some `yq` glue:

```
if [ -f "${PWD}/version-override.yml" ]; then
APP_VERSION=$(yq eval '.. | select(has("tag")).tag' version-override.yml)
else
APP_VERSION=$(yq eval '.. | select(has("tag")).tag, .. | select(has("tags")).tags[0]' vendir.yml vendir.lock.yml | head -n1)
fi
```

This is later surfaced with the Carvel Packaging API, since it requires semver to properly function.

**Describe the solution you'd like**
I later on need to populate a Package CR and it needs a semver passed into it. If vendir had some sort of command, like `vendir lock-config-tag -d dir/sub-dir` that could spit out something properly formatted for the Packaging API, I would be happy.

**Anything else you would like to add:**
I'm not sure what to do as a failover, or what happens when vendir can't ascertain what the versioning is if nothing in the folder itself is semver.

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