developer-setup: Ensure to install wheel module before all swh ones

Following the Developer setup guide makes me noticed that the wheel Python module should be installed prior all swh ones.

Indeed, the bdist_wheel command from setuptools will not be available otherwise and some packages installation will fail.

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Migrated from D1462 (view on Phabricator)

docs/developer-setup.rst

@@ -66,11 +66,10 @@ Create a virtualenv::
 
 Install all the swh packages (in develop mode)::
 
-    (swh) ~/swh-environment$ pip install $(./bin/pip-swh-packages --with-testing) \
-	                         tox pifpaf flake8
+    (swh) ~/swh-environment$ pip install flake8 pifpaf tox wheel
+    (swh) ~/swh-environment$ pip install $(./bin/pip-swh-packages --with-testing)
     [...]
 
-
 Executing unit tests
 --------------------