templater: do search for include of unqualified builtin outside of config code Commit 081b08e4ea13 (templater: look for mapfiles in template paths, 2015-05-15) added support for using things like `%include map-cmdline.default` to include built-in map files without using a valid path to them. This patch rewrites that support by moving it into `_readmapfile()` so it can later be adapted for reading from a non-file resource. Differential Revision: https://phab.mercurial-scm.org/D8792