Fix `npm dedupe` for private repositories

Fixes an issue where `npm dedupe` does not de-duplicate any private git
repositories.

Consider a tree like this:

- proj/
    package.json
    node_modules/
      proj2/
        node_modules/
          private_dep
      private_dep

Where:

* 'proj/package.json' has a dependency to 'private_dep' that is installed
   with path "git+ssh://git@github.com:acct/private_dep.git#0.1.0", e.g.:

   "dependencies": {
     "private_dep": "git+ssh://git@github.com:acct/private_dep.git#0.1.0"
   }

* proj2, which is a dependency of proj, also has a dependency to
  private_dep at the same version and path (with the exact same config
  in its package.json)

Technically 'private_dep' is the same version, but npm dedupe uses the
version in private_dep/package.json (e.g. 0.1.0) to do the resolution
rather than the path.

This code will use the git paths in node_modules/ package.json and
the calculated '_from' field to do the comparisons:

  "_from":  "git+ssh://git@github.com:acct/private_dep.git#0.1.0"