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>

solver/socketprovider_unix.go

@@ -4,17 +4,17 @@
 package solver
 
 import (
-	"fmt"
+	"errors"
 	"net"
-	"strings"
 	"time"
+
+	"go.dagger.io/dagger/plancontext"
 )
 
-func dialStream(id string) (net.Conn, error) {
-	if !strings.HasPrefix(id, unixPrefix) {
-		return nil, fmt.Errorf("invalid socket forward key %s", id)
+func dialService(service *plancontext.Service) (net.Conn, error) {
+	if service.Unix == "" {
+		return nil, errors.New("unsupported socket type")
 	}
 
-	id = strings.TrimPrefix(id, unixPrefix)
-	return net.DialTimeout("unix", id, time.Second)
+	return net.DialTimeout("unix", service.Unix, time.Second)
 }