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/dom/requirement.cppm

@@ -1,11 +1,11 @@
-/// @module ftxui.dom.requirement
-/// @brief Module file for the Requirement struct of the Dom module
+/// @module ftxui.dom:Requirement
+/// @brief Module file for the Requirement struct of the DOM module
 
 module;
 
 #include <ftxui/dom/requirement.hpp>
 
-export module ftxui.dom.requirement;
+export module ftxui.dom:Requirement;
 
 /**
  * @namespace ftxui