Conflicting fields are ignored in deps specified by location

[Ericson2314]

1. Within git deps, it is possible to specify all three of branch, tag, and revision:

   let reference = details.branch.clone().map(GitReference::Branch)
       .or_else(|| details.tag.clone().map(GitReference::Tag))
       .or_else(|| details.rev.clone().map(GitReference::Rev))
       .unwrap_or_else(|| GitReference::Branch("master".to_string()));

2. It is possible to specify git and path, in which case git is used

   let new_source_id = match details.git {
       Some(ref git) => {
           ...
       }
       None => {
           match details.path.as_ref() {
               Some(path) => {
                   ...
               }
               None => None,
           }
       }
   }

We should add a warning or make this an error. I learn towards error because any package that uses this is already violating the author's intention.

[alexcrichton]

I'm going to tag this as E-easy as it should be relatively easy to detect this in the manifest parser, and it should also be pretty easy to add a warning.

We'll want to add a warning for at least a few cycles, and maybe after a few months we can turn it into a hard error once we're certain it's not used. For now though we can just lint against this happening.