Refactor BuildGraphLoader::load().

Move the code that actually loads the project from a file into its own
function.

Change-Id: I258f97e8411a54e99fa274041f9054ce0703e273
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
remove deprecated Product.additionalFileTags

Change-Id: I98b4ddc32266eaa83ece5ed4ba814d183206671e
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>