Use module partitions instead of full modules (#1146)

Follow-up to #1015. This pull request replaces the full modules that represent headers, with partitions, to emphasise the belonging of the header to the module. This should hopefully provide a speedup to compilation, and confuse users less by aggregating the usable modules into a smaller set.

src/ftxui/screen/color.cppm

@@ -1,11 +1,11 @@
-/// @module ftxui.screen.color
+/// @module ftxui.screen:Color
 /// @brief Module file for the Color class of the Screen module
 
 module;
 
 #include <ftxui/screen/color.hpp>
 
-export module ftxui.screen.color;
+export module ftxui.screen:Color;
 
 /**
  * @namespace ftxui
@@ -14,6 +14,10 @@ export module ftxui.screen.color;
 export namespace ftxui {
     using ftxui::Color;
 
+    /**
+     * @namespace literals
+     * @brief The FTXUI ftxui::literals:: namespace
+     */
     inline namespace literals {
         using ftxui::literals::operator""_rgb;
     }