templater: switch to lower-level config.parse() in _readmapfile()
I hope to modify this code to also work with resources loaded from
memory (for PyOxidizer support). For that, we'll need to handle the
lookup of relative %include path (not joined with the base directory
of the containing file). This patch prepares for that by using
config.parse() instead of config.read(), since the latter expects
a file in the file system.
As it happens, this change also lets us clean up the config class to
not need the _includepaths field. That will happen next.
Differential Revision: https://phab.mercurial-scm.org/D8791