feat: add JULIA_PKG_GC_AUTO env var

[pfitzseb]

This PR adds a JULIA_PKG_DISABLE_AUTO_GC environment variable to allow users to disable automatic garbage collection without reaching into Pkg's internals and setting _auto_gc_enabled[] = false manually.

[ericphanson]

This should get documented here too: https://docs.julialang.org/en/v1/manual/environment-variables/#Pkg.jl

[pfitzseb]

-> JuliaLang/julia#51583

[ericphanson]

looks good to me! it could be nice to dynamically check before doing auto GC's rather than setting it in __init__ so one can set the ENV variable later. But this already seems an improvement so IMO that shouldn't block merging.

[staticfloat]

so one can set the ENV variable later.

I think the general pattern of ENV["JULIA_PKG_xxx"] = "false" in scripts should be discouraged, and Pkg._auto_gc_enabled[] = false should be encouraged for these reasons:

• Environment variables are not thread safe and so should generally be thought of as a communication mechanism from outside of Julia to inside of Julia, not communication for things within Julia itself.
• Priority can be difficult to determine; if ENV["JULIA_PKG_DISABLE_AUTO_GC"] is set to 1, but then the user later sets _auto_gc_enabled[] = true, which one wins? In the current setup, this is clear (_auto_gc_enabled[] is the ground truth, it copies the value of ENV["JULIA_PKG_DISABLE_AUTO_GC"] at __init__() and then the value is thereafter ignored).

[pfitzseb]

Yeah, that second point is also what I was mostly concerned about.
Might be worth adding a function to set _auto_gc_enabled without touching that very internal looking ref, e.g. Pkg.API.auto_gc(::Bool). Thoughts?

[fingolfin]

This was approved almost a year ago, but not merged. What gives?

[DilumAluthge]

@staticfloat Is this good to merge once CI passes?

[DilumAluthge]

Also bump @pfitzseb

[fredrikekre]

Should it be JULIA_PKG_AUTO_GC=true/false instead?

[IanButterworth]

Yeah like JULIA_PKG_PRECOMPILE_AUTO=false

[IanButterworth]

Or JULIA_PKG_GC_AUTO to match the order

[pfitzseb]

Renamed the env var and added API.auto_gc function for getting and setting the _auto_gc_enabled state.