Simplify .Site.GetPage etc.

This commit is a follow up to a recent overhaul of the GetPage/ref/relref implemenation.

The most important change in this commit is the update to `.Site.GetPage`:

* To reduce the amount of breakage in the wild to its minimum, I have reworked .Site.GetPage with some rules:

* We cannot support more than 2 arguments, i.e. .Site.GetPage "page" "posts" "mypage.md" will now throw an error. I think this is the most uncommon syntax and should be OK. It is an easy fix to change the above to .Site.GetPage "/posts/mypage.md" or similar.
* .Site.GetPage "home", .Site.GetPage "home" "" and .Site.GetPage "home" "/" will give you the home page. This means that if you have page in root with the name home.md you need to do .Site.GetPage "/home.md" or similar

This commit also fixes some multilingual issues, most notable it is now possible to do cross-language ref/relref lookups by prepending the language code to the path, e.g. `/jp/posts/mypage.md`.

This commit also reverts the site building tests related to this to "Hugo 0.44 state", to get better control of the changes made.

Closes #4147
Closes #4727
Closes #4728
Closes #4728
Closes #4726
Closes #4652

hugolib/pages_language_merge_test.go

@@ -68,8 +68,8 @@ func TestMergeLanguages(t *testing.T) {
 	assert.Equal(4, len(firstNN.Sites()))
 	assert.Equal("en", firstNN.Sites().First().Language.Lang)
 
-	nnBundle, _ := nnSite.getPage(nil, "bundle")
-	enBundle, _ := enSite.getPage(nil, "bundle")
+	nnBundle := nnSite.getPage("page", "bundle")
+	enBundle := enSite.getPage("page", "bundle")
 
 	assert.Equal(6, len(enBundle.Resources))
 	assert.Equal(2, len(nnBundle.Resources))