__package_pip: add optional dependencies

This is a poor implementation of optional dependencies for pip packages.
It ensures to install them if the package will be installed, but does
not take into account if they must be added/removed after the package is
already installed. Also, it will not be autoremoved, as all dependencies
will not be removed.
3 jobs for feature/__package_pip/extras in 1 minute and 10 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Test
passed #7776
pycodestyle

00:00:16

failed #7775
shellcheck

00:00:28

passed #7777
unit_tests

00:00:25

 
Name Stage Failure
failed
shellcheck Test
Removing cdist/util/__pycache__/
Removing cdist/version.py

Skipping Git submodules setup
Authenticating with credentials from job payload (GitLab Registry)
$ ./bin/cdist-build-helper version
$ ./bin/cdist-build-helper shellcheck
cdist/conf/type/__package_pip/gencode-remote:61:19: note: read without -r will mangle backslashes. [SC2162]
ERROR: Job failed: exit code 1