fix: replace subprocess which with shutil.which() in Process.exists()

The previous approach shelled out to the `which` binary via Popen, which
has three problems:
- `which` is not present on all systems (Alpine, minimal Docker images),
  causing an unhandled FileNotFoundError on every exists() call
- any non-path text on stdout (e.g. zsh built-in printing "foo not found"
  to stdout) produced a false positive
- spawning a subprocess for a PATH lookup is unnecessary overhead

shutil is already imported and shutil.which() is already used by
_resolve_tool() in the same class, as well as system_check.py and
dependency.py. Using it here is consistent and correct on all platforms.

Author: vlkumiloslearning

wifite/util/process.py

@@ -200,11 +200,7 @@ def exists(program):
         if Configuration.initialized and program in Configuration.existing_commands:
             return Configuration.existing_commands[program]
 
-        p2 = Process(['which', program])
-        stdout = p2.stdout().strip()
-        stderr = p2.stderr().strip()
-
-        exist = bool(stdout)
+        exist = shutil.which(program) is not None
         if Configuration.initialized:
             Configuration.existing_commands.update({program: exist})
         return exist