FIX Change build approach to "double-compile"

Previous approaches and why they don't work/are not nice:

* symlink to piqi_src on post-get-deps hook:

    post_hook for get-deps gets executed after THIS project is retrieved, not
    after its dependencies are (see [rebar issue]), therefore the symlinking
    fails.

* symlink to piqi_src on pre-compile hook:

    the first thing rebar does is scan and consult `.app.src` files, so this will
    cause the "fake" `.app.src` file to be used, because it will ignore the new
    `.app.src` file in the (later) symlinked folder

* copy over .app file after compile

  - the file is static, so you will need to manually maintain modules, rebar
    does a better job at this using .app.src files
  - you also need to manually replace `{git, vsn}`

New approach: symlink on pre-compile, recompile twice on the very first compile
to ensure the right `.app.src` file was used.

[rebar issue]: https://github.com/rebar/rebar/issues/61