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
public attribute not declared in interfaces: badpeer.badattribute
public attribute not declared in interfaces: badpeer.badmethod