Support "local" directive in component.json.

Duo currently ignores the "local" and "paths" entries in component.json when resolving dependencies.  This change resolves that by adding an isLocal test when resolving.  It appends the dependency to all of the paths listed in the "paths" entry (and also tries root), returning the first one for which fs.exists reports true.

The isLocal test *only* gets used when component.json includes a "locals" entry.  Otherwise, dependency resolution remains unchanged.
@stephenmathieson Did you mean fs.exists specifically?  I switched over to co-exists since I noticed Duo is already using that.