cmake: Remove defaults for COMPONENT_SRCDIRS, COMPONENT_SRCS, COMPONENT_ADD_INCLUDEDIRS

* Philosophical: "explicit is better than implicit".
* Practical: Allows useful errors if invalid directories given in components as the defaults aren't
  always used. Also trims the -I path from a number of components that have no actual include
  directory.
* Simplifies knowing which components will be header-only and which won't


* Original commit: espressif/esp-idf@4f1a856

Author: projectgus

components/mdns/CMakeLists.txt

@@ -1,2 +1,5 @@
+set(COMPONENT_SRCDIRS ".")
+set(COMPONENT_ADD_INCLUDEDIRS "include")
+set(COMPONENT_PRIV_INCLUDEDIRS "private_include")
 register_component()