grunt: Minor clean up in buildloader task

* Rename src/dest to target/template.
  The src/dest properties are special in grunt and trigger
  the built-in file task utilities (such as pattern expansion
  and also turns them into an array if not already).
  Our task, however, doesn't support multiple sources or
  destinations and was bypassing it (not this.files but directly
  as this.data.src).
  To avoid confusion (a grunt user looking at this Gruntfile would
  assume it is okay to use an array and patterns in src/dest),
  and to reduce a small amount of overhead in grunt populating
  this.files for us, use different names instead.

* Remove hardcoded "__dirname + '/../../'" prefix from the
  file.read() call. It defaults to expanding relative to the
  Gruntfile root. Hardcoding this made the task not re-usable
  from the MediaWiki extension repository that submodulese this
  one (and includes this task from lib/ve).

Change-Id: I1b897f0a7479dfee1d46875eee6bda649f74e943