Allow PyB projects to be installed via URL-based install

[arcivanov]

Currently PyB allows itself to be installed via pip install <url> by means of a setup file. Unfortunately other projects using PyB cannot take advantage of this without copying the file.

Therefore,

• See if setup.py can be cleaned up and/or simplified. Simplified in Optional task exclusion doesn't function correctly #269
• Add a task into the core that (re-)generates setup.py file to provide URL-based install to any project.

[mriehl]

Good points. Maybe we can integrate this with the scaffolding module (pyb --start-project). Kinda like when starting a scala project you get the option of embedding a SBT launcher, also very much like the gradle wrapper which is created by idea when you set up a gradle project.

[arcivanov]

Yep, --start-project seems like a reasonable place for this. But we also need to allow regenerating of the file if we have to refresh it. Can --start-project be rerun idempotently without breaking what's already there?

[mriehl]

No, it's pretty basic templating. We would have to go through the AST somehow in order to regenerate, but even then it's not easy because project.set_property() does not look like getattr(project, "set_property")() in the AST but it's actually the same thing. So it would be really hard to not re-add things that are already there.

Maybe we can have both: a task that (re)generates the setup.py and --start-project simply calls this when it's done.

[arcivanov]

Yeah, I'm for the latter approach I think.

[arcivanov]

depends on #263