Allow simultaneous path and url under [sources] with path-priority

[thchr]

I have a situation where I'd like to use the [sources] entry of Project.toml, to indicate that a package, X, should first be looked for at some path - and, if not present there, be looked for at some url. I had hoped this might be possible to do by something like

[sources]
X = {path="..", url="https://github.com/user/X.jl"}

but that's not possible currently, by design, cf.

Pkg.jl/src/project.jl

Lines 104 to 106 in 69c6de0

	if haskey(source, "path") && (haskey(source, "url") || haskey(source, "rev"))
	pkgerror("Both `path` and `url` or `rev` are specified in `source` section")
	end

I think it would be nice to be able to do this though: it would allow one to develop multiple interdependent private packages locally, preferring local versions specified in path if the exist, and then distribute them to collaborators, who might not have a local versions (then pulling from url).

Basically, I think it would be natural to let path take precedence if both path and url are present and a directory with a project exists at path.