Fix resolution of middleware modules

Fix the code loading "loopback#errorhandler" (for example) to correctly
look up the "loopback" module in node_modules of the bootstrapped
application, instead of looking it up in node_modules of loopback-boot.

lib/compiler.js

@@ -569,7 +569,7 @@ function resolveMiddlewarePath(rootDir, middleware, config) {
 
   // Try to require the module and check if <module>.<fragment> is a valid
   // function
-  var m = require(pathName);
+  var m = require(sourceFile);
   if (typeof m[fragment] === 'function') {
     resolved.sourceFile = sourceFile;
     return resolved;