extensions: move the "import" logic out from "load"

extensions: move the "import" logic out from "load"

The "load" method does too many things: on-demand import and check version.
This patch moves the import logic out from "load" so it could be wrapped to
change the import behavior, for example, chg will use it to pre-import
extensions.

--- a/mercurial/extensions.py	Fri Oct 07 07:43:04 2016 -0400
+++ b/mercurial/extensions.py	Mon Oct 03 03:37:10 2016 +0100
@@ -80,6 +80,26 @@
         mod = getattr(mod, comp)
     return mod
 
+def _importext(name, path=None, reportfunc=None):
+    if path:
+        # the module will be loaded in sys.modules
+        # choose an unique name so that it doesn't
+        # conflicts with other modules
+        mod = loadpath(path, 'hgext.%s' % name)
+    else:
+        try:
+            mod = _importh("hgext.%s" % name)
+        except ImportError as err:
+            if reportfunc:
+                reportfunc(err, "hgext.%s" % name, "hgext3rd.%s" % name)
+            try:
+                mod = _importh("hgext3rd.%s" % name)
+            except ImportError as err:
+                if reportfunc:
+                    reportfunc(err, "hgext3rd.%s" % name, name)
+                mod = _importh(name)
+    return mod
+
 def _reportimporterror(ui, err, failed, next):
     # note: this ui.debug happens before --debug is processed,
     #       Use --config ui.debug=1 to see them.
@@ -98,21 +118,7 @@
     if shortname in _extensions:
         return _extensions[shortname]
     _extensions[shortname] = None
-    if path:
-        # the module will be loaded in sys.modules
-        # choose an unique name so that it doesn't
-        # conflicts with other modules
-        mod = loadpath(path, 'hgext.%s' % name)
-    else:
-        try:
-            mod = _importh("hgext.%s" % name)
-        except ImportError as err:
-            _reportimporterror(ui, err, "hgext.%s" % name, "hgext3rd.%s" % name)
-            try:
-                mod = _importh("hgext3rd.%s" % name)
-            except ImportError as err:
-                _reportimporterror(ui, err, "hgext3rd.%s" % name, name)
-                mod = _importh(name)
+    mod = _importext(name, path, bind(_reportimporterror, ui))
 
     # Before we do anything with the extension, check against minimum stated
     # compatibility. This gives extension authors a mechanism to have their