Make Pkg operations case sensitive on all platforms?

[jakebolewski]

Follow up to #13542. Miscaptializing package names should error here (on OSX).

julia> Pkg.clone("distributions")
INFO: Cloning distributions from git://github.com/JuliaStats/Distributions.jl.git
ERROR: Base.Pkg.PkgError("distributions already exists")
 [inlined code] from pkg.jl:13
 in clone at pkg/entry.jl:193
 in clone at pkg/entry.jl:221
 in anonymous at pkg/dir.jl:31
 in cd at file.jl:22
 in cd at pkg/dir.jl:31
 [inlined code] from pkg/dir.jl:25
 in clone at pkg.jl:37
 in eval at ./boot.jl:264

[stevengj]

+1.

[jakebolewski]

Closing as a dupe of #17747.

[tkelman]

should've been the other way around, but fair enough