[ty] Check source roots/namespaces are setup correctly

[sureshjoshi]

Re: pantsbuild/pants#22952 (comment)

[aviau]

Recap of my original problem:

I could find some targets where it seems that ty does not find the modules that are being imported.

After some investigating, I could fix it by adding the following in my pyproject.toml:

[tool.ty.environment]
root = [
  "./root1",
  "./root2",
]

It would be nice if the plugin found a way to auto-fill this using pants roots.

[aviau]

I noticed another problem with this actually. If you have a ty root that points to a folder that isn't copied in the sandbox, it will raise an error.

example:

[tool.ty.environment]
root = ["./src", "./lib", "./vendor"]

If src, lib or vendor is missing, ty will raise an error.

ty failed
  Cause: /Users/alexandre.viau/git/example/src does not point to a directory

Not sure what the best pants way of fixing this. I would say one of:

1. Generate the root config ourselves, avoiding the problem from hapening because we only configure roots that are present in the sandbox.
2. Always make sure to at least create empty directories for each root in the ty config.