TemporaryPipInstallResolver: Prevent pip install from polluting stdout

I noticed this while playing around with --install-deps and --json, and
at the same time piping the JSON output into `jq` for filtering/colors:
The `pip install` that is triggered by --install-deps will print status
messages to both stdout and stderr, and since these are not handled by
our subprocess.run() call, they will appear directly on _our_
stdout/stderr. When piping our stdout to `jq` this causes `jq` to get
those status messages (preceding the JSON output) on its stdin, and it
predictably aborts with an error message.

This commit fixes the issue in two ways:

First, we pass --quiet and --disable-pip-version-check to `pip install`,
to silence some common/unnecesary output. This seems sufficient to make
`pip install` silent when everything works as expected. However, when
warnings and errors might still be printed in exceptional cases.

Next, we capture the stdout + stderr from `pip install` and only print
them in case the `pip install` command failed.

With both of these fixes, we ensure that `pip install` only makes noise
when it fails, and that noise only appears on our stderr (in the shape
of a warning log message). For example, if I try to --install-deps with
an intentional typo ("pyyamll):

  WARNING:fawltydeps.packages:Command failed: ['/run/user/1000/tmpr5l6m94i/bin/pip', 'install', '--no-deps', '--quiet', '--disable-pip-version-check', 'pyyamll', 'scikit-learn']
  WARNING:fawltydeps.packages:Output:
  ERROR: Could not find a version that satisfies the requirement pyyamll (from versions: none)
  ERROR: No matching distribution found for pyyamll

  WARNING:fawltydeps.packages:Failed to install 'pyyamll'
  WARNING:fawltydeps.packages:Output:
  ERROR: Could not find a version that satisfies the requirement pyyamll (from versions: none)
  ERROR: No matching distribution found for pyyamll

  ERROR:fawltydeps.main:Unresolved dependencies: pyyamll
  FawltyDeps is unable to find the above packages with the configured package resolvers. Consider using --pyenv if these packages are already installed somewhere, or --custom-mapping-file to take full control of the package-to-import-names mapping.

fawltydeps/packages.py

@@ -376,15 +376,33 @@ def installed_requirements(
         marker_file = venv_dir / ".installed"
         if not marker_file.is_file():
             venv.create(venv_dir, clear=True, with_pip=True)
-        argv = [f"{venv_dir}/bin/pip", "install", "--no-deps"]
-        proc = subprocess.run(argv + requirements, check=False)
+        # Capture output from `pip install` to prevent polluting our own stdout
+        pip_install_runner = partial(
+            subprocess.run,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.STDOUT,
+            text=True,
+            check=False,
+        )
+        argv = [
+            f"{venv_dir}/bin/pip",
+            "install",
+            "--no-deps",
+            "--quiet",
+            "--disable-pip-version-check",
+        ]
+        proc = pip_install_runner(argv + requirements)
         if proc.returncode:  # pip install failed
             logger.warning("Command failed: %s", argv + requirements)
+            if proc.stdout.strip():
+                logger.warning("Output:\n%s", proc.stdout)
             logger.info("Retrying each requirement individually...")
             for req in requirements:
-                proc = subprocess.run(argv + [req], check=False)
+                proc = pip_install_runner(argv + [req])
                 if proc.returncode:  # pip install failed
                     logger.warning("Failed to install %s", repr(req))
+                    if proc.stdout.strip():
+                        logger.warning("Output:\n%s", proc.stdout)
         marker_file.touch()
         yield venv_dir