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. Do a shallow copy of the whole
value instead, which will copy the `loadFunc` field too.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I9f456fc7f09497ef16ce5d09a2f84c1f48d3374e
Dispatch-Trailer: {"type":"trybot","CL":1201522,"patchset":1,"ref":"refs/changes/22/1201522/1","targetBranch":"master"}

cue/load/loader_common.go

@@ -243,15 +243,13 @@ 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,
-			}
+			// Copy the original instance including private fields,
+			// because the private fields include the loadFunc, without which
+			// the import logic will not work.
+			q0 := *p
+			q = &q0
+			q.PkgName = pkg
+			q.ImportPath += ":" + pkg
 			fp.pkgs[pkg] = q
 		}
 		p = q

cue/load/loader_test.go

@@ -635,8 +635,10 @@ root:   $CWD/testdata/testmod
 dir:    $CWD/testdata/testmod
 display:.
 files:
-    $CWD/testdata/testmod/test.cue`,
-	}}
+    $CWD/testdata/testmod/anon.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)