node_modules symlinks

so, I've been using this pattern of symlinking a /lib folder into node_modules so that I can always require things local to a project without specifying long relative paths (e.g. avoiding `require('../../../../some-module')`).

however, regular browserify transforms don't get applied to any files included this way because moduleDeps doesn't consider them to be top level: https://github.com/substack/module-deps/blob/master/index.js#L171

this patch addresses the issue by resolving all symlinks with `fs.realpath` before handing off to moduleDeps.

although I've been able to work around this by using globalTransforms, they are much slower than the simple realpath check for projects with many dependencies.