prepare the transition to #Plan.context

This change helps the transition between `dagger input` and `#Plan.context`.

In summary, the codebase now relies on a *context* for execution with mapping to *IDs*.
In the future, *context* will come from a `#Plan.context`.
In the meantime, a bridge converts `dagger input` to a plan context. This allows both *old* and *new* style configurations to co-exist with the same underlying engine.

- Implement `plancontext`. Context holds the execution context for a plan. Currently this includes the platform, local directories, secrets and services (e.g. unix/npipe).
- Contextual data can be registered at any point. In the future, this will be done by `#Plan.context`
- Migrated the `dagger input` codebase to register inputs in a `plancontext`
- Migrated low-level types/operations to the *Context ID* pattern.
  - `dagger.#Stream` now only includes an `id` (instead of `unix` path)
  - `dagger.#Secret` still includes only an ID, but now it's based off `plancontext`
  - `op.#Local` now only includes an `id` (instead of `path`, `include`, `exclude`.

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

stdlib/dagger/dagger.cue

@@ -18,13 +18,7 @@ import (
 #Stream: {
 	@dagger(stream)
 
-	{
-		// Unix Socket
-		unix: string
-	} | {
-		// Windows Named Pipe
-		npipe: string
-	}
+	id: string
 }
 
 // Secret value

stdlib/dagger/op/op.cue

@@ -20,10 +20,8 @@ package op
 }
 
 #Local: {
-	do:  "local"
-	dir: string
-	include: [...string]
-	exclude: [...string]
+	do: "local"
+	id: string
 }
 
 // FIXME: bring back load (more efficient than copy)