Respect singleVersionForAllModules for added modules

[mkavanagh]

Modules with no previous artifact were not being written into the changes file, so they were not being kept up to date even when singleVersionForAllModules was enabled.

This caused project versions to get out of sync if a new module was added to the project at the same time as any API changes were made to other modules.

[codecov]

Codecov Report

Merging #179 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #179      +/-   ##
============================================
- Coverage     73.48%   73.47%   -0.02%     
  Complexity      149      149              
============================================
  Files           175      175              
  Lines          8992     8992              
  Branches       1501     1501              
============================================
- Hits           6608     6607       -1     
+ Misses         1803     1802       -1     
- Partials        581      583       +2     

Impacted Files	Coverage Δ	Complexity Δ
...org/revapi/maven/AbstractVersionModifyingMojo.java	75.64% <100.00%> (+0.51%)	0.00 <0.00> (ø)
...org/revapi/java/JavaElementDifferenceAnalyzer.java	79.92% <0.00%> (-0.71%)	0.00% <0.00%> (ø%)

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[metlos]

Nice one! Thanks for also adding the integration test!