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...

requested review from @douardda

assigned to @olasd

mentioned in commit olasd/swh-loader-core@27433d1a

Jenkins job DTPL/gitlab-builds #63 succeeded .
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mentioned in merge request swh-loader-core!520 (closed)

mentioned in merge request swh-core!373 (closed)

approved this merge request

LGTM

merged