ui: represent paths as classes

Many have long wanted for paths to have expanded functionality and
flexibility.

In order to make that transition possible, we need to start
representing paths as something more than simple strings.

This patch introduces two classes:

1) "path" for representing a single path instance
2) "paths" for representing a collection of "paths"

Since we don't like patches that introduce new code without any
consumers, we convert ui.expandpath() to use the new APIs internally.
Upcoming patches will start exposing "path" instances to consumers
that currently interface with string paths.

The new "paths" attribute of ui is populated from config data the first
time it is accessed. Since it isn't updated when the configs are
modified, this could lead to some inaccurate caching behavior. It
shouldn't be an issue, as paths information is typically not accessed
until command dispatch, which occurs after the repository config
and extensions have been loaded. Time will tell if we need to refresh
paths information when the underlying config changes.

mercurial/ui.py

@@ -531,10 +531,14 b' class ui(object):'
         if util.hasscheme(loc) or os.path.isdir(os.path.join(loc, '.hg')):
             return loc
 
-        path = self.config('paths', loc)
-        if not path and default is not None:
-            path = self.config('paths', default)
-        return path or loc
+        p = self.paths.getpath(loc, default=default)
+        if p:
+            return p.loc
+        return loc
+
+    @util.propertycache
+    def paths(self):
+        return paths(self)
 
     def pushbuffer(self, error=False):
         """install a buffer to capture standard output of the ui object
@@ -923,3 +927,48 b' class ui(object):'
         ui.write(ui.label(s, 'label')).
         '''
         return msg
+
+class paths(dict):
+    """Represents a collection of paths and their configs.
+
+    Data is initially derived from ui instances and the config files they have
+    loaded.
+    """
+    def __init__(self, ui):
+        dict.__init__(self)
+
+        for name, loc in ui.configitems('paths'):
+            # No location is the same as not existing.
+            if not loc:
+                continue
+            self[name] = path(name, rawloc=loc)
+
+    def getpath(self, name, default=None):
+        """Return a ``path`` for the specified name, falling back to a default.
+
+        Returns the first of ``name`` or ``default`` that is present, or None
+        if neither is present.
+        """
+        try:
+            return self[name]
+        except KeyError:
+            if default is not None:
+                try:
+                    return self[default]
+                except KeyError:
+                    pass
+
+        return None
+
+class path(object):
+    """Represents an individual path and its configuration."""
+
+    def __init__(self, name, rawloc=None):
+        """Construct a path from its config options.
+
+        ``name`` is the symbolic name of the path.
+        ``rawloc`` is the raw location, as defined in the config.
+        """
+        self.name = name
+        # We'll do more intelligent things with rawloc in the future.
+        self.loc = rawloc