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
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true