Support building colcon extensions with no setup.py (#14)

This change separates the installation of the package being tested from
the installation of its dependencies. After initial installation, the
package's name is found using `pip list` and excluded from the org-level
constraints file that's used to ensure that HEAD is used for each of
those repositories.

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

@@ -17,9 +17,14 @@ runs:
         python -m pip install -U pip setuptools
         echo ::endgroup::
 
+        echo ::group::Install package
+        python -m pip install -U -e .[test] --no-deps
+        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 list --local --editable --format json | jq '.[0].name' -r)
+        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::