If an environment is being imported then don't allow deletion

[pierskarsenbarg]

Two parts to this:

1. If you have two environments (env-a and env-b), and you import env-a into env-b and then delete env-a, you don't get any warning that it's being imported and then you can't open, run, or indeed get any values out of env-b.
2. If we know that a stack has an environment attached (because it's part of the stack config and has done an update) then stop the environment being deleted until it has been removed from the stack config and a further update has occurred.

[arunkumar611]

The first step is to provide insights into those dependencies.

The second step is to allow for user controls such as this: https://github.com/pulumi/esc/issues/358