Support 'exclusive' interfaces

[emdoyle]

Now that interfaces can be 'split' using visibility, it is useful to be able to say that a given interface is the 'exclusive' way that a module is allowed to be used by another module.

For example, consider the following config within a tach.domain.toml:

[[modules]]
path = "api"
depends_on = ["**"]
visibility = ["<domain_root>", "//module5"]
layer = "mid"

[[interfaces]]
expose = ["something"]
from = ["api"]

[[interfaces]]
expose = ["MyAPI"]
from = ["api"]
visibility = ["//module5"]
exclusive = true

Given the configuration above, the 'default' interface exposed by api is api.something.
However, module5 may only use api.MyAPI.

Before this PR, and without the exclusive flag, this would not have been possible. In that case, module5 would have been allowed to use both api.something and api.MyAPI.