tox.ini: use "develop" mode for test environment installation

In combination with the Pytest 8.1 changes for namespace package support, this allows us to get rid of the envsitepackagesdir hacks, which makes the output of all tools much more useful.

Tested on swh.core, swh.scheduler, and swh.loader.core, which make extensive use of entrypoints and module name matching...

copier-template/tox.ini.jinja

@@ -7,20 +7,17 @@ envlist =
   py3
 
 [testenv]
+usedevelop = true
 extras =
   testing
 deps =
   pytest-cov
 commands =
   pytest --doctest-modules \
-         --rootdir {envsitepackagesdir} \
-         --cov={envsitepackagesdir}/{{ package_root }} \
+         --cov={{ package_root }} \
          --cov-branch \
-         {envsitepackagesdir}/{{ package_root }} \
+         {{ package_root }} \
          {posargs}
-# --rootdir (with --import-mode from pytest.ini) are required to make tests
-# that depends on the test file to be a proper submodule of the swh namespace
-# after migration to PEP420 (implicit namespace).
 
 [testenv:black]
 skip_install = true