`pulumi query` is designed, essentially, as a souped-up `exec`. We
execute a query program, and add a few convenience constructs (e.g., the
default providers that give you access to things like `getStack`).
Early in the design process, we decided to not re-use the `up`/update
path, both to minimize risk to update operations, and to simplify the
implementation.
This commit will add this "parallel query universe" into the engine
package. In particular, this includes:
* `QuerySource`, which executes the language provider running the query
program, and providing it with some simple constructs, such as the
default provider, which provides access to `getStack`. This is much
like a very simplified `EvalSource`, though notably without any of the
planning/step execution machinery.
* `queryResmon`, which disallows all resource operations, except the
`Invoke` that retrieves the resource outputs of some stack's last
snapshot. This is much like a simplified `resmon`, but without any of
the provider resolution, and without and support for resource
operations generally.
* Various static functions that pull together miscellaneous things
needed to execute a query program. Notably, this includes gathering
language plugins.
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