Split and refactor requirements.txt into multiple files

[Raruto]

Motivation

Allowing docker developers to add a single line inside their entrypoint.sh in order to easily reinstall all necessary python dependencies (without rebuilding a new image), eg:

# g3w-suite-docker/scripts/docker-entrypoint-dev.sh 

if [ ! -e "/shared-volume/build_done" ]; then
  pip3 install -r /code/requirements.txt
fi

List of changes

Refactor the main requirements.txt so it always installs all necessary dependencies (including them from all other files listed here)

# Django
-r ./g3w-admin/core/requirements.txt

# Huey
-r ./g3w-admin/caching/requirements_huey.txt

# Caching
-r ./g3w-admin/caching/requirements.txt

# File manager
-r ./g3w-admin/filemanager/requirements.txt

# Qplotly
-r ./g3w-admin/qplotly/requirements.txt

# Openrouteservice
-r ./g3w-admin/openrouteservice/requirements.txt

Alternative solution

If those dependencies are only useful to docker users, perhaps it would be better to just refactor the requirements_docker.txt file as described above.

Related to: #544 (comment)