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>

plancontext/context.go

@@ -0,0 +1,48 @@
+package plancontext
+
+import (
+	"crypto/sha256"
+	"encoding/json"
+	"fmt"
+)
+
+type ContextKey string
+
+// Context holds the execution context for a plan.
+//
+// Usage:
+// ctx := plancontext.New()
+// id := ctx.Secrets.Register("mysecret")
+// secret := ctx.Secrets.Get(id)
+type Context struct {
+	Platform    *platformContext
+	Directories *directoryContext
+	Secrets     *secretContext
+	Services    *serviceContext
+}
+
+func New() *Context {
+	return &Context{
+		Platform: &platformContext{
+			platform: defaultPlatform,
+		},
+		Directories: &directoryContext{
+			store: make(map[ContextKey]*Directory),
+		},
+		Secrets: &secretContext{
+			store: make(map[ContextKey]*Secret),
+		},
+		Services: &serviceContext{
+			store: make(map[ContextKey]*Service),
+		},
+	}
+}
+
+func hashID(v interface{}) ContextKey {
+	data, err := json.Marshal(v)
+	if err != nil {
+		panic(err)
+	}
+	hash := sha256.Sum256(data)
+	return ContextKey(fmt.Sprintf("%x", hash))
+}