PyPI: generate optional plugins as separate source tarballs

Now that the optional plugins have been excluded from the main
git archive, let's create them (currently with setuptools sdist
command).

Also, let's adjust the upload instruction a bit to signal that
other packages may be in PYPI_UPLOAD too.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

Makefile

@@ -73,6 +73,14 @@ source-release: clean
 source-pypi: clean
 	if test ! -d PYPI_UPLOAD; then mkdir PYPI_UPLOAD; fi
 	git archive --format="tar" --prefix="avocado-framework/" $(VERSION) | tar --file - --delete 'avocado-framework/optional_plugins' > "PYPI_UPLOAD/avocado-framework-$(VERSION).tar"
+	for PLUGIN in $(AVOCADO_OPTIONAL_PLUGINS); do\
+		if test -f $$PLUGIN/setup.py; then\
+			echo ">> Creating source distribution for $$PLUGIN";\
+			cd $$PLUGIN;\
+			$(PYTHON) setup.py sdist -d ../../PYPI_UPLOAD;\
+			cd -;\
+                fi;\
+	done
 
 pypi: source-pypi develop
 	mkdir PYPI_UPLOAD/avocado-framework
@@ -86,7 +94,7 @@ pypi: source-pypi develop
 	@echo "The URL to do that may be a bit tricky to find, so here it is:"
 	@echo " https://pypi.python.org/pypi?%3Aaction=submit_form"
 	@echo
-	@echo "Alternatively, you can also run: "
+	@echo "Alternatively, you can also run a command like: "
 	@echo " twine upload -u <PYPI_USERNAME> PYPI_UPLOAD/avocado-framework-$(VERSION).tar.gz"
 	@echo