dagger.#FS support

- Implement dagger.#FS support
- Migrate `context.imports` to dagger.#FS
- Backward compat: dagger.#FS can be passed in lieu of a
  dagger.#Artifact
- For instance, an import (`dagger.#FS`) can be passed to the current
  `yarn.#Package` implementation

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

plancontext/localdir.go

@@ -0,0 +1,60 @@
+package plancontext
+
+import (
+	"path/filepath"
+	"sync"
+)
+
+type LocalDir struct {
+	Path string
+}
+
+type localDirContext struct {
+	l     sync.RWMutex
+	store map[ContextKey]*LocalDir
+}
+
+func (c *localDirContext) Register(directory *LocalDir) ContextKey {
+	c.l.Lock()
+	defer c.l.Unlock()
+
+	id := hashID(directory)
+	c.store[id] = directory
+	return id
+}
+
+func (c *localDirContext) Get(id ContextKey) *LocalDir {
+	c.l.RLock()
+	defer c.l.RUnlock()
+
+	return c.store[id]
+}
+
+func (c *localDirContext) List() []*LocalDir {
+	c.l.RLock()
+	defer c.l.RUnlock()
+
+	directories := make([]*LocalDir, 0, len(c.store))
+	for _, d := range c.store {
+		directories = append(directories, d)
+	}
+
+	return directories
+}
+
+func (c *localDirContext) Paths() (map[string]string, error) {
+	c.l.RLock()
+	defer c.l.RUnlock()
+
+	directories := make(map[string]string)
+	for _, d := range c.store {
+		abs, err := filepath.Abs(d.Path)
+		if err != nil {
+			return nil, err
+		}
+
+		directories[d.Path] = abs
+	}
+
+	return directories, nil
+}