support hex organizations for igniter.install

[hypno2000]

Is your feature request related to a problem? Please describe.
Majority of our source code is private and published to hex under our organization. Right now igniter.install does not work for our own hex packages.

Describe the solution you'd like
A possibility to specify optional organization prefix when installing packages with igniter.install.

Describe alternatives you've considered
We could use git dependencies for those specific packages but that would mean we cannot use hex for our other packages that depend on those as well as one cant publish packages to hex that has git dependencies.

Express the feature either with a change to resource syntax, or with a change to the resource interface

For example something like:

mix igniter.install foo acme:bar acme:baz

Or

mix igniter.install foo acme/bar acme/baz

[zachallaun]

Could definitely be supported. I personally like repo/package, which would work fine with all the existing package formats:

mix igniter.install repo/package
mix igniter.install repo/package@version
mix igniter.install repo/package@git:url
mix igniter.install repo/package@github:user/repo
mix igniter.install repo/package@path:path

repo:package would work just as well, but for whatever reason, I prefer the slash.

[hypno2000]

I think slash makes more sense as well. Package names in hex.pm are also presented in org/package format.