plugin-webpack: fixes / improvements to default splitChunks config

[mxmul]

Changes

• Fixes two bugs with the "large lib" chunks.
  1. Searches /web_modules rather than /node_modules for large third-party JS modules.
  2. Sets a unique chunk name for each library; the current config had all large modules going into a single vendor chunk, which wasn't the intent.
• Decrease the size threshold for extracting large modules into a "large lib" chunk to 100KB. The compact: true setting in our babel-loader config compressed files a bit before getting to splitChunks, and allowed some pretty large libraries (like Moment) to get under the 150KB threshold.
• Pass the correct numEntries value to getSplitChunksConfig
• Disable the commons chunk entirely when there is only one entry point.

Testing

Manually tested against a small project that has one entrypoint and imports Moment (a large JS dependency). Before, all modules were pulled into a commons chunk and Moment wasn't pulled out into its own chunk; after this patch, the commons chunk is gone and lib-moment.HASH.js is emitted.

I also tested adding a second entry point to the project, and it introduces a commons chunk as expected:

Docs

None; bug fixes only.

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[FredKSchott]

LGTM! All changes make sense (appreciate the detailed explanation)