absolute-paths-not-starting-with-crate warning emitted for macros from 2015 edition crates used in 2018 edition crates

[sdroege]

See e.g.

warning: absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
   --> src/lib.rs:156:1
    |
156 | / gst_plugin_define!(
157 | |     ndi,
158 | |     env!("CARGO_PKG_DESCRIPTION"),
159 | |     plugin_init,
...   |
165 | |     env!("BUILD_REL_DATE")
166 | | );
    | |__^
    |
    = note: `-W absolute-paths-not-starting-with-crate` implied by `-W rust-2018-compatibility`
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
    = note: for more information, see issue #53130 <https://github.com/rust-lang/rust/issues/53130>
    = note: this warning originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

The macro in question is declared by a 2015 edition crate and used in a 2018 edition crate.

Right now this is only annoying but if this ever becomes a hard error then using all 2015 crates as dependencies of an 2018 crate will be impossible, which was one of to promises of the editions system.

[Centril]

cc @petrochenkov