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