Asset directory emission, test refactoring

[guybedford]

Fixes #68.

This extends asset emission to direct emission of directories, fixing the strong-globalize test case.

I also went ahead and refactored the tests to use the "run" approach over an embedded eval. This simplifies the source maps handling but did require rewriting out the module.exports execution. Top-level promises work out here fine though since as a separate process, each test runs to completion.

This does not close #58 as it is effectively just the first step of that.

[codecov-io]

Codecov Report

Merging #106 into master will increase coverage by 1.75%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage   73.31%   75.06%   +1.75%     
==========================================
  Files           6        6              
  Lines         356      385      +29     
==========================================
+ Hits          261      289      +28     
- Misses         95       96       +1

Impacted Files	Coverage Δ
src/index.js	93.33% <100%> (+1.8%)	⬆️
src/loaders/relocate-loader.js	92.6% <93.54%> (+0.03%)	⬆️

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[gijo-varghese]

@guybedford I'm getting the error with the latest version. I've updated in #68