Path resolution semantics should be hygenic

[djrenren]

Macros written in 2018 edition that are expanded within 2015 edition crates, should still get to use 2018 path resolution.

That is, if the macro foo! expands to ::random::prelude::random() this should work when invoked in 2015 even if random was not previously mentioned via extern crate

[eddyb]

cc @joshtriplett @petrochenkov

[joshtriplett]

cc @aturon

[petrochenkov]

Duplicate of #52848, #50376
cc #50999

[petrochenkov]

Closed by #56053