build: Install local editable after installing requirements.

I would add the local project to dev.in but that results in the compiled
output including a fully qualified path to my local version of the
project right now which is not helpful.

Adding `-e .` to `dev.in` puts a `-e file://my/local/path/to/sample-plugin`
in the dev.txt file which is not helpful.

Author: feanil

backend/Makefile

@@ -67,6 +67,8 @@ piptools: ## install pinned version of pip-compile and pip-sync
 
 requirements: clean_tox piptools ## install development environment requirements
 	pip-sync -q requirements/dev.txt requirements/private.*
+	# So that the plugin entrypoints are installed and loaded correctly.
+	pip install -e .
 
 test: clean ## run tests in the current virtualenv
 	pytest