Support "out-of-tree" stdlib development

[vchuravy]

On the ci-dev call we discussed the possible to use Revise.jl for testing stdlibs.
For that we would need to specify a rewrite rule that maps from the usr/share location to a local development version of the stdlib.

@giordano @staticfloat @DilumAluthge @gbaraldi

[staticfloat]

Thinking out loud here about API, my ideal API would be something like a "module replacement" API, where I can do something like:

using Pkg
Revise.track(Pkg => path_to_pkg_on_disk)

[KristofferC]

How would that work when changing structs is not supported?