Fix scan_pkg issue with cycle over-detection

[vtjnash]

The main intent here is to ensure that could_be_cycle[pkg] = false happens even when called from the root package, and not just recursively from scan_deps

[Keno]

I don't understand why this patch isn't a no-op. On what path does this change behavior?

[vtjnash]

For the root, I didn't see where it would get set to false eventually in could_be_cycle, but I could have just missed it

[Keno]

It should get set on return from scan_deps!. I mean, I may have messed it up, but just from what I can tell, this PR is literally equivalent.

[vtjnash]

Doesn't the root of the search call scan_pkg and not call scan_deps? I also might not be reading that bit right, this change is purely by visual inspection

[IanButterworth]

I'm going to hold off backporting #3651 to 1.9 until this is resolved

[Keno]

Doesn't the root of the search call scan_pkg and not call scan_deps? I also might not be reading that bit right, this change is purely by visual inspection

scan_deps! always gets called on the first call to scan_pkg. Again, as far as I can tell, this PR is a no-op.

[vtjnash]

Ah, okay, right. I was confused by having a mutating helper method in a conditional if statement, and that wasn't processing in my mind as being executed always. This PR might remove the need for Box-ing of the two functions for the mutual recursion pairing, but they are functionally equivalent.

[IanButterworth]

It'd be tidy to resolve this before moving this code to Base.

On a fully precompiled env with DiffEq

This PR

julia> @time Pkg.precompile()
  0.333714 seconds (579.76 k allocations: 77.397 MiB, 1.35% gc time)

julia> @time Pkg.precompile()
  0.340852 seconds (579.76 k allocations: 77.397 MiB, 2.02% gc time)

master

julia> @time Pkg.precompile()
  0.416346 seconds (579.03 k allocations: 77.364 MiB, 23.80% gc time)

julia> @time Pkg.precompile()
  0.332330 seconds (579.03 k allocations: 77.364 MiB)

[KristofferC]

they are functionally equivalent.

So I don't think this really needs to be "resolved".