Relocate fuzzer files and the explorer_fuzzer rule. (#3017)

Putting fuzzer files under //testing to emphasize the testonly aspect
(consolidates bazel and common subdirectories). The attributes on
explorer_fuzzer are also a little skewed from what's desirable; it's
been working okay, but this should still be a refinement.

explorer/fuzzing/fuzzer_util_test.cpp

@@ -9,9 +9,9 @@
 
 #include <fstream>
 
-#include "common/fuzzing/proto_to_carbon.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/raw_ostream.h"
+#include "testing/fuzzing/proto_to_carbon.h"
 
 namespace Carbon::Testing {
 namespace {