CLI Coverage

[guybedford]

This implements coverage for the CLI by making the integration tests run through the cli driver directly, instead of only using the internal API.

Under this model, we use the API for the unit tests, and the CLI for the integration tests.

In order to get this to work easily with the coverage, I simply modified the CLI file to return an API interface when running in a non-CLI mode. This also required fixing the behaviour of the require.main === module check which should help many other build cases as well :)

Coverage is now around 80%, and we can continue to improve this still quite easily as well.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@04fbbe7). Click here to learn what that means.
The diff coverage is 51.18%.

@@            Coverage Diff            @@
##             master     #251   +/-   ##
=========================================
  Coverage          ?   81.45%           
=========================================
  Files             ?       16           
  Lines             ?      685           
  Branches          ?        0           
=========================================
  Hits              ?      558           
  Misses            ?      127           
  Partials          ?        0

Impacted Files	Coverage Δ
src/index.js	69.01% <ø> (ø)
src/loaders/relocate-loader.js	97.14% <100%> (ø)
src/cli.js	59.47% <47.45%> (ø)

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