Treat uglify-js as empty by default

[guybedford]

This fixes the pug build, by always ignoring uglify-js builds. I know it doesn't sound ideal, but that really is the best fix we can come up with here given there isn't an analysis solution with the current technology.

When this happens the following warning is output:

ncc: Ignoring build of uglify-js, as it is not statically analyzable. Build with "--external uglify-js" if this package is needed.

[rauchg]

I think this is a good solution for now, considering the popularity of the module

[codecov-io]

Codecov Report

Merging #307 into master will increase coverage by 1.61%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #307      +/-   ##
==========================================
+ Coverage   66.76%   68.37%   +1.61%     
==========================================
  Files          11       12       +1     
  Lines         352      370      +18     
==========================================
+ Hits          235      253      +18     
  Misses        117      117

Impacted Files	Coverage Δ
src/index.js	69.53% <ø> (ø)	⬆️
src/loaders/empty-loader.js	100% <100%> (ø)

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