bot failed to bucket many labels into `lib / src`

[Fishrock123]

See nodejs/node#12366

cc @mscdex

[phillipj]

This is because lib / src label bucket/limiter is exclusively done for files in lib/. Most interesting code is in matchSubSystemsByRegex() and hasJsSubsystemChanges().

I take it that's wrong, so I started on a fix: #141