rewrite generate_parent_navs to work on resource paths

the old code was kind of impossible to understand by reading it, so this
is hopefully considerably clearer

tests/test_pages.py

@@ -24,32 +24,26 @@ def test_resolve_page_file_other_requests_fine():
 def test_generate_page_navs_index(app):
     """Test that the index page has navs to the root (itself)."""
     with app.app_context():
-        assert generate_parent_navs('/') == [('incorporeal.org', '/')]
-
-
-def test_generate_page_navs_alternate_index(app):
-    """Test that the index page (as a page, not a path) also has navs only to the root (by path)."""
-    with app.app_context():
-        assert generate_parent_navs('index') == [('incorporeal.org', '/')]
+        assert generate_parent_navs('pages/index.md') == [('incorporeal.org', '/')]
 
 
 def test_generate_page_navs_subdir_index(app):
     """Test that dir pages have navs to the root and themselves."""
     with app.app_context():
-        assert generate_parent_navs('subdir/') == [('incorporeal.org', '/'), ('/subdir/', '/subdir/')]
+        assert generate_parent_navs('pages/subdir/index.md') == [('incorporeal.org', '/'), ('/subdir/', '/subdir/')]
 
 
 def test_generate_page_navs_subdir_real_page(app):
     """Test that real pages have navs to the root, their parent, and themselves."""
     with app.app_context():
-        assert generate_parent_navs('subdir/page') == [('incorporeal.org', '/'), ('/subdir/', '/subdir/'),
-                                                       ('Page', '/subdir/page')]
+        assert generate_parent_navs('pages/subdir/page.md') == [('incorporeal.org', '/'), ('/subdir/', '/subdir/'),
+                                                                ('Page', '/subdir/page')]
 
 
 def test_generate_page_navs_subdir_with_title_parsing_real_page(app):
     """Test that title metadata is used in the nav text."""
     with app.app_context():
-        assert generate_parent_navs('subdir-with-title/page') == [
+        assert generate_parent_navs('pages/subdir-with-title/page.md') == [
             ('incorporeal.org', '/'),
             ('SUB!', '/subdir-with-title/'),
             ('/subdir-with-title/page', '/subdir-with-title/page')