Usage '[patch.*]` report

[EPashkin]

@GuillaumeGomez, @sdroege
I tried use [patch.crates-io] in gtk-rs crates to minimize difference between master and crate branches.
See my test forks https://github.com/EPashkin/examples/tree/as_master,
https://github.com/EPashkin/rust-gnome-sys/tree/as_crate,
https://github.com/EPashkin/glib/tree/as_crate,
https://github.com/EPashkin/cairo/tree/as_crate,
https://github.com/EPashkin/gtk/tree/as_crate
As I used only part crates there there too many [path.*] section in Cargo.toml,
in gtk-rs IMHO it will be only [patch.crates-io].

It works fine until I tried emulate glib version upgrade from 0.6.0 to 0.7.0:
examples tries build both current and next version,
as there already many cons found I stopped checks.

Pros: difference only [patch.crates-io] section,
also as this sections ignored in dependencies IMHO possible just publish crate from master branch

Cons:

1. Totally incompatible with .cargo\config\path: cargo not allow mixing.
2. If someone want use git version then he need use [patch.crates-io] too.
3. Related errors unclear: in next error "0.7.0" is version of conflicting sys (not sure which)

error: failed to select a version for `gtk`.
    ... required by package `gtk-rs-examples v0.0.1 (file:///D:/eap/rust/0/examples)`
versions that meet the requirements `^0` are: 0.7.0

the package `gtk-rs-examples` depends on `gtk`, with features: `v3_22, v3_16, v3_20, v3_10, v3_22_30, v3_18` but `gtk` does not have these features.

[GuillaumeGomez]

Still seems like a huge hack and the errors are clearly an issue from my point of view. Besides this, it seems very promising!