checkpoint for downlevel-dts

[glasser]

This is an attempt at addressing #6423. However, it sure seems like typesVersions is kinda janky and doesn't work as well as we'd like. One particular thing I've found is that it doesn't play well with the ../dist lines we have in our package.json files, because it only gets applied if the previously resolved path is under the directory it is in: https://github.com/microsoft/TypeScript/blob/7b764164ede080afff02ec731dfea72b3f4f73f3/src/compiler/moduleNameResolver.ts#L1942

(Yeah, the typesVersions clauses are not consistent across all of the files here: I was doing a test that only looked at the top-level and /standalone ones so those are the ones I was iterating on.)

It's possible this is fixable but maybe we should just bump the TS requirement to v4.7.

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[glasser]

Giving up on this.