Rework externals for perf

[guybedford]

It turns out one of the expensive parts of the build was this function (easy to see by just commenting out the externals and comparing build speeds).

This reimplements the same logic using some custom optimizations.

The other thing implemented here is to move the .ts and .tsx checks in the resolver to the end. This does mean that test.js will take precedence over test.ts, but the benefit is that the Webpack resolver doesn't have to stat every single that is ever required in the tree as its .ts variation first (which it was doing for EVERYTHING in node_modules...)

Combined these shave about 5 seconds off a 40 second build for me.

[codecov-io]

Codecov Report

Merging #176 into master will increase coverage by 0.73%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #176      +/-   ##
==========================================
+ Coverage   77.29%   78.03%   +0.73%     
==========================================
  Files          11       11              
  Lines         511      519       +8     
==========================================
+ Hits          395      405      +10     
+ Misses        116      114       -2

Impacted Files	Coverage Δ
src/index.js	95.95% <100%> (+2.48%)	⬆️
src/utils/get-package-base.js	100% <100%> (ø)	⬆️

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