Add link to generate a virtual env under plz-out

[Tatskaari]

No description provided.

[samwestmoreland]

How come one's a dlink and the other a link? Is it something about retaining the directory structure?

[Tatskaari]

How come one's a dlink and the other a link? Is it something about retaining the directory structure?

The srcs rules output files in slightly different ways. The wheel rules produce {module}/thing.py whereas the pip rules produce {name}/{module}/thing.py.

The dlink label removes the extra level of nesting.

[afterthought]

@Tatskaari, I tested this and it doesn't quite work. Only 1 pip_library gets linked to python/venv and then the remaining ones are skipped because this line is checking if the destination exists and it does already after the first lib creates python/venv.