Reuse typechecking results - stage 1

[psfinaki]

First part of the implementation of reusing typecheck results, as per the design doc.

This does the following:

• adds the flag --reusetypecheckingresults, with all the hooking
• detects if there is a case to reuse typechecking results

Nothing happens - yet - we only report related activities though.

Cache invalidation scenarios I looked at:

• changed sources
• changed references
• changed command line

[github-actions]

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[psfinaki]

@T-Gro PTAL, this is an early stage so I mostly just wanted to discuss the questions I raised in the comments, so that we go in the agreed direction.

[edgarfgp]

It seems like this is going to rely/build on top of graph checking have couple questions:

• Would this PR Turn a bunch of options by default #17442 be included ?
• If we are going to rely on this would be important to enable by default ?

[psfinaki]

@edgarfgp so we'll need to discuss this - personally I'd like to turn graph-checking on. Now (like, before Xmas) is a rather good time - we can catch some bugs there before the next big release.

It's somewhat scaring but, well, just this probably shouldn't be stopping us 👀 Graph-checking and reusing typechecking results can test each other and I'd say we should use this benefit here.

I am not speaking for the whole team just yet - only sharing some thoughts. We will discuss and prioritize this.

[psfinaki]

Merging, since this goes to a feature branch now.