rcutil: move scmutil.*rcpath to rcutil (API) As discussed at [1], the logic around "actual config"s seem to be non-trivial enough that it's worth a new module. This patch creates the module and move "scmutil.*rcpath" functions there as the first step. More methods will be moved to the module in the future. The module is different from config.py because the latter only cares about data structure and parsing, and does not care about special case, or system config paths, or environment variables. [1]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-March/095503.html