packaging: get the version from the lsv distribution; declare coverage

[ArjunS07]

Purpose

This fork installs as the lsv distribution. Three places ask importlib.metadata for the version of the asv distribution. When only lsv is installed, that lookup raises an error and import asv fails. The lightspeed commands also import coverage, which is not a declared dependency. This PR asks for the version of lsv and adds coverage to the dependencies.

Changes

1. Version lookup uses lsv (asv/__init__.py, asv/commands/common_args.py, asv/commands/publish.py).
   Effect: import asv and asv --version work when only lsv is installed.
   Before: importlib_metadata.PackageNotFoundError: No package metadata was found for asv.
2. coverage is a dependency (pyproject.toml).
   Effect: a clean install can import the lightspeed commands.
   Before: KeyError: 'InitializeDiffcheck' from the plugin loader when coverage was missing.

Verification

Clean Python 3.12 venv, pip install -e ".[test]" of this branch:

• python -c "import asv": works, version 0.1.dev2850+gfc16ba472; python -m asv --version prints the same.
• pytest test/test_check.py test/test_util.py: 49 passed. On main the same venv cannot import asv.

Notes

• Independent of the lightspeed dependency fix; merge in either order.
• Environments that also have the upstream asv installed (the task images) now show the lsv version instead of the upstream one.