Add a cache to file path mapping

[berickson1]

This greatly reduces the time needed to resolve FilePathKey objects on watch builds. (~20% from 20s to 16s)

Note: The only case where I could see this optimization performing poorly is if a symlink was changed between builds, but I don't believe webpack handles that gracefully anyways.

Relates to #1215

See the performance traces below:

Before:

After:

[johnnyreilly]

Haha - nice visuals! Did you make those by following the debugging instructions here: https://github.com/TypeStrong/ts-loader/blob/master/CONTRIBUTING.md#debugging-ts-loader-installed-from-npm-in-your-own-webpack-project ? Or was it something else?

[johnnyreilly]

I've one naming nit but otherwise this LGTM. Do you want to add an entry to the CHANGELOG.md and increment the package.json please?

[berickson1]

Haha - nice visuals! Did you make those by following the debugging instructions here: https://github.com/TypeStrong/ts-loader/blob/master/CONTRIBUTING.md#debugging-ts-loader-installed-from-npm-in-your-own-webpack-project ? Or was it something else?

Oh neat, I didn't realize that you had debugging instructions anywhere. I used a combination of node --inspect and ndb to profile and do some initial investigations

[johnnyreilly]

Yeah I think @andrewbranch put the debug instructions together. Looking at this PR makes me think it would be great to have some perf testing instructions as well... Could you share what your workflow looked like to produce the above?

[berickson1]

Yeah I think @andrewbranch put the debug instructions together. Looking at this PR makes me think it would be great to have some perf testing instructions as well... Could you share what your workflow looked like to produce the above?

Added in #1230

[johnnyreilly]

Thanks! https://github.com/TypeStrong/ts-loader/releases/tag/v8.0.13 is shipping now. 🚢