gluon-core: check-site: support checking "custom" values

The new "value" helper can be used to turn a Lua value into a path that
can be passed to need_*() etc.

.luacheckrc

@@ -25,6 +25,7 @@ files["package/**/check_site.lua"] = {
 		"extend",
 		"in_domain",
 		"in_site",
+		"value",
 		"need",
 		"need_alphanumeric_key",
 		"need_array",

package/gluon-core/luasrc/lib/gluon/check-site.lua

@@ -57,6 +57,10 @@ end
 
 
 local function path_to_string(path)
+	if path.is_value then
+		return path.label
+	end
+
 	return table.concat(path, '.')
 end
 
@@ -96,6 +100,10 @@ local function domain_src()
 end
 
 local function conf_src(path)
+	if path.is_value then
+		return 'Configuration'
+	end
+
 	local src
 
 	if has_domains then
@@ -138,6 +146,14 @@ function M.in_domain(path)
 	return path
 end
 
+function M.value(label, value)
+	return {
+		is_value = true,
+		label = label,
+		value = value,
+	}
+end
+
 function M.this_domain()
 	return domain_code
 end
@@ -171,6 +187,10 @@ function loadpath(path, base, c, ...)
 end
 
 local function loadvar(path)
+	if path.is_value then
+		return path.value
+	end
+
 	return loadpath({}, conf, unpack(path))
 end