Support building colcon extensions with no setup.py

This change uses a pip "download" operation to read the name of the
package being built so that it can be removed from the constraints.txt
prior to downloading dependencies.

The package name must be removed from constraints.txt or pip will fail
to install the package and its dependencies due to the conflict.

Note that pytest-cov still requires a setup.cfg to be present - storing
coverage information in pyproject.toml is not yet supported by this
action.

action.yaml

@@ -14,12 +14,14 @@ runs:
         else
           . $VENV/bin/activate
         fi
-        python -m pip install -U pip setuptools
+        python -m pip install -U pip setuptools wheel
         echo ::endgroup::
 
         echo ::group::Install dependencies
         # Remove this package from constraints
-        grep -v "^$(python setup.py --name)@" ${GITHUB_ACTION_PATH}/constraints.txt > constraints.txt
+        PKG_NAME=$(pip download . --no-deps --pre --no-build-isolation |  grep -i 'Successfully downloaded' | awk '{ print $NF }')
+        echo "Removing ${PKG_NAME} from constraints.txt"
+        grep -v "^${PKG_NAME}@" ${GITHUB_ACTION_PATH}/constraints.txt > constraints.txt
         # Install dependencies, including any 'test' extras, as well as pytest-cov
         python -m pip install -U -e .[test] pytest-cov -c constraints.txt
         echo ::endgroup::