Improve affected module detection

[zinserjan]

Improves the changed module detection algorithm:

• respect code splitting (e.g. require.ensure)
• run all tests that were affected by a file change (no need to run all tests until all tests are fixed)

other improvements:

• support multiple output files (e.g. produced by multiple entry points or require.ensure)
• clear require cache only for changed files
• no longer inject changed files into the test bundle

[codecov-io]

Current coverage is 86.93% (diff: 92.22%)

Merging #86 into master will increase coverage by 1.48%

@@             master        #86   diff @@
==========================================
  Files            18         19     +1   
  Lines           474        505    +31   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits            405        439    +34   
+ Misses           69         66     -3   
  Partials          0          0          

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