Reduce asset emission and handle unexpected core assets

[guybedford]

I just noticed the core build was giving the assets emitted error.

This was because:

• The directory emission was resulting in a lot of new ".js" assets being emitted (eg the whole of webpack/lib was being emitted as an asset), when these wouldn't be supported in those workflows, so instead disabled ".js" emission for directory assets. This should reduce build sizes nicely.
• The directory emission is actually correctly handling the TypeScript declaration files that we have a custom copy for, which was nice to see
• There are some locales being emitted by one of the core dependencies, that we don't use, so I'm just filtering this out.

[guybedford]

I've disabled the loopback test for now as the middleware is broken as in #141, this PR just makes that error happen earlier.

[codecov-io]

Codecov Report

Merging #166 into master will decrease coverage by 0.19%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #166     +/-   ##
=========================================
- Coverage   77.49%   77.29%   -0.2%     
=========================================
  Files          11       11             
  Lines         511      511             
=========================================
- Hits          396      395      -1     
- Misses        115      116      +1

Impacted Files	Coverage Δ
src/loaders/relocate-loader.js	92.77% <100%> (-0.39%)	⬇️

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