Add repository generation via cookiecutter (frequenz-floss#39)

- Fix use of optional dependencies
- Improve documentation
- Don't raise if there are no dependencies
- Revert "Use sets for config paths and sessions"
- Deduplicate config paths
- Add cookiecutter template to generate repositories

Fixes frequenz-floss#21, fixes frequenz-floss#11.

Author: llucax

README.md

@@ -5,6 +5,40 @@ repository for Frequenz projects.
 
 If offers:
 
+* [Cookiecutter] templates for scaffolding new projects
 * Trivial build of `noxfile.py` with some predefined sessions with all common
   checks.
 * Tools to build protobuf/grpc files as Python, including type information.
+
+
+[Cookiecutter]: https://cookiecutter.readthedocs.io/en/stable
+
+## Start a new projects
+
+To start a new project you should first [install
+Cookiecutter](https://cookiecutter.readthedocs.io/en/stable/installation.html).
+It is normally available in any Linux distribution, but some have a very old
+version (for example, Ubuntu/Debian).  You can [check which version your distro
+has in Repology](https://repology.org/project/cookiecutter/versions). You need
+**at least version 2.1.0**.  To make sure to get an up to date version you can
+always uses `pip` and install in a `venv`:
+
+```console
+$ python -m venv cookiecutter
+$ cd cookiecutter
+$ . bin/activate
+[cookiecutter] $ pip install cookiecutter
+Collecting cookiecutter
+...
+```
+
+Then just run cookiecutter where you want to create the new project. A new
+directory will be created with the generated project name. For example:
+
+```sh
+cd ~/devel
+cookiecutter gh:frequenz-floss/frequenz-repo-config-python --directory=cookiecutter
+```
+
+This will prompt for the project type, name and other configuration and
+generate the whole project for you.

cookiecutter/cookiecutter.json

@@ -0,0 +1,29 @@
+{
+	"type": [
+		"actor",
+		"api",
+		"app",
+		"lib",
+		"model"
+	],
+	"name": null,
+	"description": null,
+	"keywords": "(comma separated: 'frequenz', <type> and <name> are included automatically)",
+	"github_org": "frequenz-floss",
+	"license": [
+		"MIT",
+		"Proprietary"
+	],
+	"author_name": "Frequenz Energy-as-a-Service GmbH",
+	"author_email": "[email protected]",
+	"python_package": "{{cookiecutter | python_package}}",
+	"pypi_package_name": "{{cookiecutter | pypi_package_name}}",
+	"github_repo_name": "{{cookiecutter | github_repo_name}}",
+	"_extensions": [
+		"jinja2_time.TimeExtension",
+		"local_extensions.github_repo_name",
+		"local_extensions.keywords",
+		"local_extensions.pypi_package_name",
+		"local_extensions.python_package"
+	]
+}