Fix loading modules with AMD defines

[devongovett]

Some UMD modules like moment-timezone have their AMD check before the CommonJS one. When an AMD loader like RequireJS is on the page, this causes the module to call define instead of exposing the exports to Parcel via CommonJS.

This PR sets define to undefined inside modules so that the AMD check will not pass and the exports will be exposed to CommonJS instead.

[codecov-io]

Codecov Report

Merging #1285 into master will increase coverage by 0.25%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1285      +/-   ##
==========================================
+ Coverage   89.03%   89.28%   +0.25%     
==========================================
  Files          80       80              
  Lines        4577     4544      -33     
==========================================
- Hits         4075     4057      -18     
+ Misses        502      487      -15

Impacted Files	Coverage Δ
src/visitors/globals.js	94.73% <100%> (+0.29%)	⬆️
src/assets/JSAsset.js	95.68% <100%> (ø)	⬆️
src/worker.js	95.23% <0%> (-4.77%)	⬇️
src/assets/CSSAsset.js	82.72% <0%> (-4.55%)	⬇️
src/Logger.js	95.78% <0%> (-1.06%)	⬇️
src/utils/loadPlugins.js	90.47% <0%> (+11.9%)	⬆️
src/assets/WebManifestAsset.js	78.57% <0%> (+19.38%)	⬆️

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