internal/core/export: suport default picking

This introduces default picking for during export.

This is needed to implement the Final option
for Value.Syntax correctly.

Fixes #489

Change-Id: Ie2a4d687c3ce66c2f1c5c7e53b1138c257ec85ab
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6889
Reviewed-by: CUE cueckoo <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mpvl

cue/types.go

@@ -927,6 +927,7 @@ func (v Value) Syntax(opts ...Option) ast.Node {
 
 	p := export.Profile{
 		Simplify:        !o.raw,
+		TakeDefaults:    o.final,
 		ShowOptional:    !o.omitOptional && !o.concrete,
 		ShowDefinitions: !o.omitDefinitions && !o.concrete,
 		ShowHidden:      !o.omitHidden && !o.concrete,