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/platform.go

@@ -0,0 +1,32 @@
+package plancontext
+
+import (
+	"github.com/containerd/containerd/platforms"
+	specs "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+var (
+	// Default platform.
+	// FIXME: This should be auto detected using buildkit
+	defaultPlatform = specs.Platform{
+		OS:           "linux",
+		Architecture: "amd64",
+	}
+)
+
+type platformContext struct {
+	platform specs.Platform
+}
+
+func (c *platformContext) Get() specs.Platform {
+	return c.platform
+}
+
+func (c *platformContext) Set(platform string) error {
+	p, err := platforms.Parse(platform)
+	if err != nil {
+		return err
+	}
+	c.platform = p
+	return nil
+}