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
BasedOnStyle: LLVM
IndentWidth: 8
UseTab: ForIndentation
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false
IncludeCategories:
- Regex: '^<'
Priority: 1
- Regex: '^"'
Priority: 2