help: get helptext/ data from `resources` module if available

help: get helptext/ data from `resources` module if available

For PyOxidizer, we need to read configs using the `resources`
module. This patch makes it so we use that module if available
(i.e. Python >= 3.7). It does that by adding a new `open_resource()`
function to our `resourceutil` module.

Tested by running `$PYTHON ./hg help pager` for each $PYTHON in
{python2, python3.6, python3.7}.

Differential Revision: https://phab.mercurial-scm.org/D7622

--- a/mercurial/help.py	Thu Dec 12 10:26:09 2019 -0800
+++ b/mercurial/help.py	Thu Dec 12 12:57:13 2019 -0800
@@ -8,7 +8,6 @@
 from __future__ import absolute_import
 
 import itertools
-import os
 import re
 import textwrap
 
@@ -314,11 +313,11 @@
     """Return a delayed loader for help/topic.txt."""
 
     def loader(ui):
-        docdir = os.path.join(resourceutil.datapath, b'helptext')
+        package = b'helptext'
         if subdir:
-            docdir = os.path.join(docdir, subdir)
-        path = os.path.join(docdir, topic + b".txt")
-        doc = gettext(util.readfile(path))
+            package = b'helptext' + b'.' + subdir
+        with resourceutil.open_resource(package, topic + b'.txt') as fp:
+            doc = gettext(fp.read())
         for rewriter in helphooks.get(topic, []):
             doc = rewriter(ui, topic, doc)
         return doc

--- a/mercurial/utils/resourceutil.py	Thu Dec 12 10:26:09 2019 -0800
+++ b/mercurial/utils/resourceutil.py	Thu Dec 12 12:57:13 2019 -0800
@@ -35,3 +35,25 @@
     datapath = os.path.dirname(pycompat.sysexecutable)
 else:
     datapath = os.path.dirname(os.path.dirname(pycompat.fsencode(__file__)))
+
+try:
+    import importlib
+
+    # Force loading of the resources module
+    importlib.resources.open_binary
+
+    def open_resource(package, name):
+        package = b'mercurial.' + package
+        return importlib.resources.open_binary(
+            pycompat.sysstr(package), pycompat.sysstr(name)
+        )
+
+
+except AttributeError:
+
+    def _package_path(package):
+        return os.path.join(datapath, *package.split(b'.'))
+
+    def open_resource(package, name):
+        path = os.path.join(_package_path(package), name)
+        return open(path, 'rb')