Feature/config: allowSnapshotsFromSnapshots

### New feature, improvement proposal: `allowSnapshotsFromSnapshots`

Something like:
```bash
if project.version.endsWith("-SNAPSHOT"); then
allowSnapshots=true
else
allowSnapshots=false
```
The idea is that if we are working on a SNAPTSHOT, we can include other SNAPSHOTs (submodules for example), but when we are going to produce a RELEASE, we want to verify that we have not left any SNAPSHOTs behind.

The issue names no source files, tests, or entry points. First clarify the intended Maven Enforcer component and expected behavior for SNAPSHOT dependencies, then identify the relevant implementation and tests before defining what completion means.

Written by the indexing model from the issue text.