Reduce cache clobbering within cache_file_entry()

[staticfloat]

As things stand, the precompile cache is clobbered very regularly; A project with Foo.jl installed at version 1.2 and a project with Foo.jl dev'ed to ~/.julia/dev also at version 1.2 will clobber eachother's cached .ji files. This is particularly frustrating when you have two branches of a package checked out in two different projects that are technically at the same version; running code in both of those projects at the same time virtually ensures that you will be precompiling every time you load code.

I propose that we introduce a path parameter, so that we call cache_file_entry(pkg, path), and hash path into pkg.uuid to generate the package slug. There are only two places where we invoke cache_file_entry() and in both places we have path information readily available. While this would potentially cause a multiplication in the number of precompiled images, I think it would be worthwhile to err on the side of saving time rather than space.

[tkf]

How about the cache files for package Bar.jl depending on Foo.jl? Same version of Bar.jl may be used with different versions of Foo.jl including dev'ed version. I don't think it is enough to rely on package identity and path if you want to eliminate unnecessary precompilation. If you want to get rid of unnecessary precompilation (and save space), I think you need to generate cache paths based on something like hash tree. It would be pretty challenging considering the dynamic nature of package loading in Julia (see, e.g., #27418 (comment)).

If you want to error on the side of saving time, I think a better option would be to use per-project cache path. (See my jlm CLI that does that https://discourse.julialang.org/t/22849)

[staticfloat]

Thanks @tkf, I'm going to close this in favor of #27418, which has lots of great discussion already. I'll just go there and add my voice in with the crowd.