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>

compiler/compiler.go

@@ -24,6 +24,10 @@ func NewValue() *Value {
 	return DefaultCompiler.NewValue()
 }
 
+func NewValueWithContent(x interface{}, selectors ...cue.Selector) (*Value, error) {
+	return DefaultCompiler.NewValueWithContent(x, selectors...)
+}
+
 // FIXME can be refactored away now?
 func Wrap(v cue.Value) *Value {
 	return DefaultCompiler.Wrap(v)
@@ -80,6 +84,14 @@ func (c *Compiler) NewValue() *Value {
 	return empty
 }
 
+func (c *Compiler) NewValueWithContent(x interface{}, selectors ...cue.Selector) (*Value, error) {
+	v := c.NewValue()
+	if err := v.FillPath(cue.MakePath(selectors...), x); err != nil {
+		return nil, err
+	}
+	return v, nil
+}
+
 func (c *Compiler) Compile(name string, src string) (*Value, error) {
 	c.lock()
 	defer c.unlock()