Sort project.refs

project.refs is currently unsorted. This makes checking the file into git quite cumbersome. It would be nice if buildstream could sort the keys by element name before writing the `project.refs` file, so that we could get cleaner git diffs out of an invocation of `bst source track **.bst`

Start by tracing the `bst source track **.bst` entry point to where `project.refs` is written. Check how element names become keys, then verify that the generated file is consistently sorted and produces cleaner git diffs; the issue does not mention a test file to run.

Written by the indexing model from the issue text.