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/service.go

@@ -0,0 +1,29 @@
+package plancontext
+
+import "sync"
+
+type serviceContext struct {
+	l     sync.RWMutex
+	store map[ContextKey]*Service
+}
+
+type Service struct {
+	Unix  string
+	Npipe string
+}
+
+func (c *serviceContext) Register(service *Service) ContextKey {
+	c.l.Lock()
+	defer c.l.Unlock()
+
+	id := hashID(service)
+	c.store[id] = service
+	return id
+}
+
+func (c *serviceContext) Get(id ContextKey) *Service {
+	c.l.RLock()
+	defer c.l.RUnlock()
+
+	return c.store[id]
+}