Improve pgo builds

[Ddystopia]

• feat: allow local absolute paths
• feat: allow measuring against multiple crates

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I'm not as familiar with PGO, do I understand correctly that by running more commands, more measurements do accumulate and then get merged?

[Ddystopia]

One possible improvement would be to report error in case the string passed to --pgo is an absolute path but doesn't exists.

[ChayimFriedman2]

Do you actually need this, or just thought this is nice to have?

[Ddystopia]

Well I used it to compile RA to better suit my projects. Does this count as "I actually need this"?

[ChayimFriedman2]

Was there a measurable benefit?

[Ddystopia]

I didn't benchmark it, I'll try to do this in a span of two weeks

[ChayimFriedman2]

The reason I ask is that I suspect there won't be any perf benefit from customizing to a project (all rust projects look the same, when looking at the level of branch and cache predictions), therefore I don't want to accept this just because.

[Ddystopia]

Yeah, I get the point, but my objective is that my projects are quite different as it is either wasm front-ends or embedded software with lots of automatically generated FFI bindings and ra often just dies. I would try to benchmark it against stock ra if there would be some significant benefit.

[lnicola]

See #19585 (comment), or better, run your own numbers.