rename: replace "compton" in the codebase

leftovers:

1) config file path. Has to implement compatibility functionalities before
  we can change it.

2) links in man pages. Has to migrate the repo first.

3) _COMPTON_SHADOW, it has become a defacto standard, so we have to keep
   supporting it.

4) dbus names, undecided whether we should/could change it.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/compton.modulemap

@@ -20,8 +20,8 @@ module utils {
 module region {
   header "region.h"
 }
-module compton {
-  header "compton.h"
+module picom {
+  header "picom.h"
 }
 module types {
   header "types.h"