rename FORMER_STDLIBS -> UPGRADABLE_STDLIBS

[IanButterworth]

Calling these former stdlibs is confusing, I think.
My understanding:

• stdlib - package that ships with julia (no git-tree-sha1 in the manifest)
• upgradable stdlib - package that ships with julia, which can optionally be upgraded (gets a git-tree-sha1 if upgraded)
• former stdlibs - a package that used to be shipped with julia but isn't any longer (no packages yet, AFAIK)

If a julia 2.0 is cut, then upgradable stdlibs are primed to become former stdlibs, i.e. not shipped with julia, but before that it's confusing I think.

Also I don't believe we document that some stdlibs can be upgraded.

[ViralBShah]

This seems good to merge.

[KristofferC]

I think this name was made when these stdlibs (only DelimitedFiles) were in fact removed from the Julia repo. But that was reverted so the name here makes more sense.