Surface information on imports (such as revision) in AST

[seanyeh]

When opening an env, we only get the resolved env values. it would be nice if we could get information on the imports, such as the exact revisions used. This will be required for pulumi/pulumi#17706

Probable efforts:

• From Pat: make loading two operations: one to resolve the name in the import list entry to a loader-specific unique ID and one to load an environment's definition given its unique ID. For our purposes, the unique ID can be as simple as the environment slug + its revision. Then we could have the evaluator return the list of resolved imports (along with their original names) in the result of env loading / evaluation