hugolib: Add the last lookup variant for the GetPage index

Now, even for nun-bundles it is possible to do lookup without path or extension.

So, given `blog/my-blog-post.en.md` these lookups will succeed:

* `blog/my-blog-post.en.md`
* `blog/my-blog-post`
* `my-blog-post.en.md`
* `my-blog-post`

See #4312
See https://github.com/gohugoio/hugoDocs/issues/307

hugolib/page_bundler_test.go

@@ -80,6 +80,9 @@ func TestPageBundlerSite(t *testing.T) {
 				singlePage := s.getPage(KindPage, "a/1.md")
 
 				assert.NotNil(singlePage)
+				assert.Equal(singlePage, s.getPage("page", "a/1"))
+				assert.Equal(singlePage, s.getPage("page", "1"))
+
 				assert.Contains(singlePage.Content, "TheContent")
 
 				if ugly {

hugolib/page_collections.go

@@ -86,6 +86,9 @@ func (c *PageCollections) refreshPageCaches() {
 								_, name := path.Split(dir)
 								cache[name] = p
 								cache[dir] = p
+							} else {
+								// Again, ambigous
+								cache[translasionBaseName] = p
 							}
 
 							// We need a way to get to the current language version.