ui: refactor `readconfig()` into a form that can consume resources

The old form can't completely go away, because files outside of packages still
need to be read.  The name passed in here is a tuple of `package name, resource`
as needed by the resource API.

I like the idea of stating the config file is embedded in the executable by
listing is as `exe!package.resource`.  This would be consistent with how
`debuginstall` points to the executable for the python executable, lib, and
installed modules.  While in practice the filesystem path is available from the
backing ResourceReader when the resource is opened, it is a relative path on py2
and absolute on py3.  Further, while this would show in the `hg config` output
for each option if set as such here, it doesn't show in the `reading from...`
line when `--debug` is used.  The file isn't actually open where that prints, so
there's no way I see to get that info there.  So I opted for the simple prefix
to distinguish resources from files.

Differential Revision: https://phab.mercurial-scm.org/D7775

mercurial/ui.py

@@ -45,6 +45,7 b' from . import ('
 from .utils import (
     dateutil,
     procutil,
+    resourceutil,
     stringutil,
 )
 
@@ -424,6 +425,20 b' class ui(object):'
             )
         return False
 
+    def read_resource_config(
+        self, name, root=None, trust=False, sections=None, remap=None
+    ):
+        try:
+            fp = resourceutil.open_resource(name[0], name[1])
+        except IOError:
+            if not sections:  # ignore unless we were looking for something
+                return
+            raise
+
+        self._readconfig(
+            b'resource:%s.%s' % name, fp, root, trust, sections, remap
+        )
+
     def readconfig(
         self, filename, root=None, trust=False, sections=None, remap=None
     ):
@@ -434,6 +449,11 b' class ui(object):'
                 return
             raise
 
+        self._readconfig(filename, fp, root, trust, sections, remap)
+
+    def _readconfig(
+        self, filename, fp, root=None, trust=False, sections=None, remap=None
+    ):
         with fp:
             cfg = config.config()
             trusted = sections or trust or self._trusted(fp, filename)