Use pubtools namespace to resolve init file conflict [RHELDST-17607]

Currently, there's an issue with packaging where most of the pubtool-<lib> projects
contains __init__.py and other stuff which is supposed to belong only to pubtools.
Python namespaces are a convenient way to work around this issue.

tox.ini

@@ -9,7 +9,7 @@ whitelist_externals=sh
 [testenv:cov]
 usedevelop=true
 commands=
-	pytest --cov-report=xml --cov=pubtools {posargs}
+	pytest --cov-report=xml --cov=src/pubtools {posargs}
 
 [testenv:docs]
 deps=