Fix path handling in pip line magic (#13052)

* Fix path handling in `pip` line magic

* Remove unused import

* code style fix

* Remove `_get_full_path()` (`sys.executable` is always absolute)

* Quote executable path selectively, leaving whitespaces untouched.

* forgotten join

* reduce shlex usage - remove line split, leave only path quotation

IPython/core/magics/packaging.py

@@ -66,7 +66,9 @@ def pip(self, line):
         Usage:
           %pip install [pkgs]
         """
-        self.shell.system(' '.join([sys.executable, '-m', 'pip', line]))
+        python = shlex.quote(sys.executable)
+        self.shell.system(" ".join([python, "-m", "pip", line]))
+
         print("Note: you may need to restart the kernel to use updated packages.")
 
     @line_magic