* Switch to nyc, babel-plugin-istanbul & codecov-node for code coverage
This setup (very much like Babylon's) let us trace code coverage back to the `src/` directories of packages.
* Exclude package tests from coverage report
* fix: upgrade to version of nyc that tweaks a couple more things for babel
* fix: remove comment based on @hzoo's review
The problem with gulp pipeline is that it replaces /src/ with /lib/ in
filenames as a regex replace operation. The regex only worked for Unix
before. Here we add a 2nd regex accomodate Windows backslashes and choose
between the two by looking if we're using path.win32. The Unix regex is
changed to only match the package/*/src. This seemingly has no effect on
the build.
Fixes T6855
