Includes: Cleanup around net main and wallet

-Move from .h to .cpp: in main, net and wallet
-Remove unnecessary #include "main.h"
-Cleanup some wallet files includes

src/test/sighash_tests.cpp

@@ -5,7 +5,7 @@
 #include "consensus/validation.h"
 #include "data/sighash.json.h"
 #include "hash.h"
-#include "main.h"
+#include "main.h" // For CheckTransaction
 #include "random.h"
 #include "script/interpreter.h"
 #include "script/script.h"