cue/load: make imports work in "all package" mode

The `build.Instance` was being created directly by `cue/load`, but
that ends up with an instance that contains a nil `loadFunc`,
so imports are then ignored. Use the regular build context
to create the new instance instead.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I9f456fc7f09497ef16ce5d09a2f84c1f48d3374e
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1201522
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>

cue/load/loader_common.go

@@ -243,15 +243,12 @@ func (fp *fileProcessor) add(root string, file *build.File, mode importMode) {
 			return
 		}
 		if q == nil {
-			q = &build.Instance{
-				PkgName: pkg,
-
-				Dir:         p.Dir,
-				DisplayPath: p.DisplayPath,
-				ImportPath:  p.ImportPath + ":" + pkg,
-				Root:        p.Root,
-				Module:      p.Module,
-			}
+			q = fp.c.Context.NewInstance(p.Dir, nil)
+			q.PkgName = pkg
+			q.DisplayPath = p.DisplayPath
+			q.ImportPath = p.ImportPath + ":" + pkg
+			q.Root = p.Root
+			q.Module = p.Module
 			fp.pkgs[pkg] = q
 		}
 		p = q

cue/load/loader_test.go

@@ -635,7 +635,9 @@ root:   $CWD/testdata/testmod
 dir:    $CWD/testdata/testmod
 display:.
 files:
-    $CWD/testdata/testmod/test.cue`,
+    $CWD/testdata/testmod/test.cue
+imports:
+    mod.test/test/sub: $CWD/testdata/testmod/sub/sub.cue`,
 	}}
 	tdtest.Run(t, testCases, func(t *tdtest.T, tc *loadTest) {
 		pkgs := Instances(tc.args, tc.cfg)