load path before starter-kit up and check loadable

  Place the full expansion of the load path at the beginning of the
  starter-kit start up.  Also, adds a function to check if a package may
  be loaded from source
move function definition before first use
no package-install if source is available locally

  This commit updates all places where packages are automatically
  installed using ELPA to first check if the package is available from a
  local source directory (i.e., under the src/ directory).  This makes
  it possible for users to override the ELPA versions of packages with
  local versions from source.  E.g., with this change I'm able to run my
  own fork of magit and starter-kit-misc.org will not install magit from
  ELPA.