internal/core/runtime: don't share import cache

This was always supposed to be this way, but dropped off
the radar as a thing to do.

Fixes #1424
Fixes #1418
Issue #1414

Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>

Change-Id: I25b486d2ea1c6009b00732a113af29bd485f9df5
Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/533473
Reviewed-by: Marcel van Lohuizen <mpvl@gmail.com>

internal/core/runtime/runtime.go

@@ -46,6 +46,12 @@ func (r *Runtime) Init() {
 	if r.index != nil {
 		return
 	}
-	r.index = sharedIndex
+	r.index = newIndex()
+
+	// TODO: the builtin-specific instances will ultimately also not be
+	// shared by indexes.
+	r.index.builtinPaths = sharedIndex.builtinPaths
+	r.index.builtinShort = sharedIndex.builtinShort
+
 	r.loaded = map[*build.Instance]interface{}{}
 }