Stop bypassing Pkg resolution for version numbers

[Keno]

The version selection here:

BinaryBuilderBase.jl/src/Dependencies.jl

Lines 117 to 122 in 18e421f

	if build_version === nothing
	# Since we usually want to build against the oldest compatible
	# version, if `build_version` isn't set but `compat` is, make it
	# default to the minimum compatible version.
	build_version = minimum(VersionNumber(rng.lower.t) for rng in spec.ranges)
	end

is problematic, because it doesn't consider whether the minimum selected version is feasible. Instead, the full compat should be passed to Pkg's version resolution (if possible with a request to resolve the minimum feasible version).

This came up in JuliaPackaging/Yggdrasil#7411 which has complicated dependencies and it turns out the minimum compatible MPI version was incompatible with other dependencies.

[giordano]

You can pass the exact version number to use for the build as second argument to Dependency if that helps.

[Keno]

Yes, I fixed it that way, but this compat was selected by platforms/mpi.jl, so it was a bit of a pain.