Use new extension point for declaring rhino installable packages

[gonzalocasas]

This PR uses the changes on compas-dev/compas#614 to remove the install/uninstall scripts and instead use the new extension points.

What type of change is this?

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I added a line to the CHANGELOG.rst file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• I ran all tests on my computer and it's all green (i.e. invoke test).
• I ran lint on my computer and there are no errors (i.e. invoke lint).
• I added new functions/classes and made them available on a second-level import, e.g. compas_fab.robots.Configuration.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

[beverlylytle]

What version of compas has compas.plugins? I think you might need to update the dependency.

[gonzalocasas]

good catch! I was actually a bit surprised this passes the build, but it's only because we don't test the rhino installer on CI.

The dependency needs to be updated, but right now, there's no release of compas containing this extension point. I was hesitating whether to add (a significant chuck of) code to handle releases before and after this extension point exists but it would only be useful for a very brief amount of time, so, I think it's best to just hold this PR until there's a release of compas, update the dependency there, and keep less code around.

[gonzalocasas]

another installing question:
running python -m compas_fab.rhino.install -v 6.0 gives:

Traceback (most recent call last):
  File "C:\Anaconda3\envs\algo\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "C:\Anaconda3\envs\algo\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "c:\users\ines\documents\projects\compas_fab\src\compas_fab\rhino\install.py", line 27, in <module>
    packages = set(compas_rhino.install.INSTALLABLE_PACKAGES + INSTALLABLE_PACKAGES)
NameError: name 'compas_rhino' is not defined```
<sub>[View in Slack](https://gramaziokohler.slack.com/archives/C4DC11QVC/p1603203575014700)</sub>

[gonzalocasas]

This PR should fix the issue mentioned above ☝️

@beverlylytle what do you think? ready to merge?

[beverlylytle]

Yep.