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
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true