Caching properties to deduplicate sources

Without caching, the Resolver will duplicate the sources listed in the
`Pipfile` multiple times (since the property re-adds them every
they're called) in the arguments sent to the `pip` parser.

Removing unused command-line option `--debug`

pipenv/resolver.py

@@ -40,7 +40,6 @@ def get_parser():
         action="store",
         default=None,
     )
-    parser.add_argument("--debug", action="store_true", default=False)
     parser.add_argument("--system", action="store_true", default=False)
     parser.add_argument("--parse-only", action="store_true", default=False)
     parser.add_argument(

pipenv/utils/resolver.py

@@ -266,7 +266,7 @@ def prepare_pip_args(self, use_pep517=None, build_isolation=True):
         pip_args.extend(["--cache-dir", self.project.s.PIPENV_CACHE_DIR])
         return pip_args
 
-    @property
+    @cached_property
     def pip_args(self):
         use_pep517 = environments.get_from_env("USE_PEP517", prefix="PIP")
         build_isolation = environments.get_from_env("BUILD_ISOLATION", prefix="PIP")
@@ -298,7 +298,7 @@ def default_constraint_file(self):
         )
         return default_constraint_filename
 
-    @property
+    @cached_property
     def pip_options(self):
         pip_options, _ = self.pip_command.parser.parse_args(self.pip_args)
         pip_options.cache_dir = self.project.s.PIPENV_CACHE_DIR