general: extract the hack to warn of legacy imports and fallback to core/legacy.py

use it both in my.fbmessenger and my.reddit

if in the future any new modules need to be switched to namespace package structure with all.py it should make it easy to do

related:
- https://github.com/karlicoss/HPI/issues/12
- https://github.com/karlicoss/HPI/issues/89
- https://github.com/karlicoss/HPI/issues/102

my/reddit/__init__.py

@@ -6,36 +6,26 @@ from my.reddit import ...
 to:
 from my.reddit.all import ...
 since that allows for easier overriding using namespace packages
-https://github.com/karlicoss/HPI/issues/102
+See https://github.com/karlicoss/HPI/blob/master/doc/MODULE_DESIGN.org#allpy for more info.
 """
 
-# For now, including this here, since importing the module
-# causes .rexport to be imported, which requires rexport
+# prevent it from apprearing in modules list/doctor
+from ..core import __NOT_HPI_MODULE__
+
+# kinda annoying to keep it, but it's so legacy 'hpi module install my.reddit' works
+# needs to be on the top level (since it's extracted via ast module)
 REQUIRES = [
     'git+https://github.com/karlicoss/rexport',
 ]
 
-import re
-import traceback
 
-# some hacky traceback to inspect the current stack
-# to see if the user is using the old style of importing
-warn = False
-for f in traceback.extract_stack():
-    line = f.line or '' # just in case it's None, who knows..
+from my.core.legacy import handle_legacy_import
+is_legacy_import = handle_legacy_import(
+    parent_module_name=__name__,
+    legacy_submodule_name='rexport',
+    parent_module_path=__path__,
+)
 
-    # cover the most common ways of previously interacting with the module
-    if 'import my.reddit ' in (line + ' '):
-        warn = True
-    elif 'from my import reddit' in line:
-        warn = True
-    elif re.match(r"from my\.reddit\simport\s(comments|saved|submissions|upvoted)", line):
-        warn = True
-
-# TODO: add link to instructions to migrate
-if warn:
-    from my.core import warnings as W
-    W.high("DEPRECATED! Instead of my.reddit, import from my.reddit.all instead.")
-
-
-from .rexport import *
+if is_legacy_import:
+    # todo not sure if possible to move this into legacy.py
+    from .rexport import *