Made dynamic_lib migration path more explicit

[androm3da]

The dynamic_lib library has been deprecated in favor of contents on crates.io, but apparently libloading is a more specific direction that fits the need.

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[durka]

If the crate is good enough to recommend in the deprecation message, should it be a candidate for inclusion in the nursery?

[steveklabnik]

Yes, I would argue that this should point to the literal out of tree crate rather than picking a "better" one

[androm3da]

By "literal out of tree crate" do you mean that dynamic_lib is now hosted on crates.io? Or maybe that's just the plan?

[androm3da]

@steveklabnik Ok I will change it to dylib -- I think it wasn't in crates.io when I first looked, or I just didn't find it.

[alexcrichton]

@bors: r+ a8df425 rollup