Missing dist: package in appmap.yml raises PackageNotFoundError and stops the test run #387

Summary

If appmap.yml lists a dist: package that is not installed in the environment, the
agent raises importlib.metadata.PackageNotFoundError while loading the config. The
test run stops before any test executes and no recordings are written.

Environment

• appmap (Python agent) 3.0.1
• Python 3.10, Ubuntu, virtual environment
• pytest

Steps to reproduce

mkdir repro && cd repro
python3 -m venv .venv
.venv/bin/pip install appmap pytest
mkdir demo tests
printf 'def f():\n    return 1\n' > demo/__init__.py
printf 'from demo import f\n\ndef test_f():\n    assert f() == 1\n' > tests/test_demo.py
cat > appmap.yml <<'YML'
name: demo
language: python
appmap_dir: tmp/appmap
packages:
- path: demo
- dist: requests
YML
.venv/bin/python -c "import requests"   # ModuleNotFoundError, requests is not installed
.venv/bin/appmap-python .venv/bin/python -m pytest -q
ls tmp/appmap/pytest

Expected

The tests run and demo is recorded. The missing distribution is reported, and the run
carries on without that entry.

Actual

  File ".../site-packages/_appmap/importer.py", line 198, in do_import
  File ".../site-packages/_appmap/configuration.py", line 481, in __init__
  File ".../site-packages/_appmap/configuration.py", line 464, in matcher_of_config
  File ".../site-packages/_appmap/configuration.py", line 398, in __init__
  ...
importlib.metadata.PackageNotFoundError: No package metadata was found for requests

No test runs and tmp/appmap/pytest does not exist. DistMatcher.__init__ calls
importlib.metadata.files(dist), which raises for a distribution that is not
installed.

This happens easily when one appmap.yml is shared between environments that install
different optional dependencies.