Add option for using shared python packages

[fohrloop]

Some libraries, like pip, setuptools and wheel are almost always needed for a virtual environments. Also, these tools should be kept updated for their latest version. I addition, some users might want to always install some debugging / linting tools to new virtual environments.

Proposal

Add and option to venvlinkrc, like shared_packages, that should be empty or a list, like [pip, setuptools, wheel]. The [pip, setuptools, wheel] could be default, since probably at least these tools are needed for >90% of virtual environments.

This would give users the option to add there other packages they use (I have personal interest on adding IPython there, since I use the IPython.embed() for debugging. Maybe someone else would like to have pdbpp there, or pylint, etc.)

Preliminary working logic

• Packaging libraries ( pip, setuptools, wheel) are installed to a special virtual environment __venvlink__, which could be located in the venv_folder. This would assume users would not use the special name __venvlink__ as their virtual environment name (maybe give a warning if user tries to use it).
• Then, these packaging libraries would be added to any new virtual environment by adding a special .pth file to the newly created virtual environment (inside venv_folder). This is how for example pipx works.
• The packaging libraries should be kept up-to-date. The options are (1) Update to latest every time new virtual environment is created, like pipx does. (2) Update every time virtual environment is created, but only once per day or week, and keep a "latest_shared_libraries_check" timestamp in a special file, for example venvlinkrc. This would make creating virtual environments slightly faster, if many of them are created in a short period of time. Perhaps first option would be the included in the first implementation.
• In the future, the could be a "no-update" option like disable_shared_autoupdate, where values could be all or a list of packages. If even more fine-grained control is needed, the could be some option for disallowing auto-updates for major or minor releases (and allowing only minor or patch releases). This could be something like a list of tuples.

Other possible implementations

There would be also another way to implement similar outcome: Adding a list for packages in venvlinkrc that should be automatically installed for every new virtual environment (install_packages or something like that) The difference is that when using shared_packages, the shared packages would be only ever needed to install once; this would save some disk space. The outcome would not be entirely similar, so I guess both are needed. For example, if someone would like to always install numpy in every new virtual environment but keep the numpy packages isolated from each other (which is the purpose of virtual environments anyway). Therefore, some packages could be shared across virtual environments, and some could be auto-installed. (creating separate issue for that)