enable the ability to add local filesystem plugins

lua/dep.lua

@@ -1,5 +1,6 @@
 local logger = require('dep.log')
 local git = require('dep.git')
+local fs = require('dep.fs')
 local packager = require('dep.package')
 local h = require('dep.helpers')
 
@@ -143,7 +144,13 @@ function M.synctree(tree, cb)
 
   for _, package in pairs(tree) do
     local co = coroutine.create(function()
-      git.sync(package, done)
+      -- if the package provided prefers a local source then use the local
+      -- source instead of the git repository
+      if package.path then
+        fs:sync(package, done)
+      else
+        git.sync(package, done)
+      end
     end)
     coroutine.resume(co)
   end