Support for patches variable

[evanjs]

It would be nice if we could patch purely mavenix-managed expressions using e.g. the patches variable used in normal nix expressions.

I'm trying to patch a pom file for a project that has some pretty nasty hardcoded dependencies and build stages.

As one comment mentions, this can be accomplished by patching src.

In my case, however, src is merely a fetchGit call, so I'm unsure of the best approach for this project until something like patches is supported via mavenix.