Reimplement and simplify Hugo's template system

See #13541 for details.

Fixes #13545
Fixes #13515
Closes #7964
Closes #13365
Closes #12988
Closes #4891

tpl/tplimpl/embedded/templates/_markup/render-link.html

@@ -0,0 +1,22 @@
+{{- $u := urls.Parse .Destination -}}
+{{- $href := $u.String -}}
+{{- if strings.HasPrefix $u.String "#" -}}
+  {{- $href = printf "%s#%s" .PageInner.RelPermalink $u.Fragment -}}
+{{- else if and $href (not $u.IsAbs) -}}
+  {{- $path := strings.TrimPrefix "./" $u.Path -}}
+  {{- with or
+    ($.PageInner.GetPage $path)
+    ($.PageInner.Resources.Get $path)
+    (resources.Get $path)
+  -}}
+    {{- $href = .RelPermalink -}}
+    {{- with $u.RawQuery -}}
+      {{- $href = printf "%s?%s" $href . -}}
+    {{- end -}}
+    {{- with $u.Fragment -}}
+      {{- $href = printf "%s#%s" $href . -}}
+    {{- end -}}
+  {{- end -}}
+{{- end -}}
+<a href="{{ $href }}" {{- with .Title }} title="{{ . }}" {{- end }}>{{ .Text }}</a>
+{{- /**/ -}}