Allow workspaces to specify member search root

[b-phi]

There are a few related issues and open PRs #856, #1074, #1023.

For my use case, I think current limitation with workspace can be boiled down to this example. Given a "flat" monorepo structure like so. If foo and bar both depend on utils, but otherwise have conflicting dependencies, there is no way to configure a workspace properly in the root pyproject.toml .

libs
├── bar
│   ├── pyproject.toml
│   └── src
├── foo
│   ├── pyproject.toml
│   └── src
└── utils
    ├── pyproject.toml
    └── src
pyproject.toml

This change would allow the following structure, or other approaches with multiple workspaces per repo.

libs
├── bar
│   ├── pyproject.toml
│   └── src
├── foo
│   ├── pyproject.toml
│   └── src
└── utils
    ├── pyproject.toml
    └── src
workspaces
├── bar
│   └── pyproject.toml
└── foo
    └── pyproject.toml

My rust is quite rusty, sorry in advance.