(#220) raise CCG0007 on multi-targeting modules

[nils-a]

fixes #220

[codecov[bot]]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (23f19c7) 94.33% compared to head (cd4bc9a) 95.96%.

Files	Patch %	Lines
src/Tasks/TargetFrameworkVersions.cs	89.13%	2 Missing and 3 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #291 +/- ## =========================================== + Coverage 94.33% 95.96% +1.63% =========================================== Files 16 16 Lines 829 867 +38 Branches 111 115 +4 =========================================== + Hits 782 832 +50 + Misses 20 15 -5 + Partials 27 20 -7 ```

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