cmd/cue: add test for issue 3155

This test will fail if the package resolution logic scans all packages
in the module, as reported in https://cuelang.org/issue/3155.

We add the test now so we can see that it's fixed in a subsequent CL.

For #3155.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I57d320cb0f4fdf656fdd5f5f41d1cf150fe6813d

cmd/cue/cmd/testdata/script/pkg_resolution_focus.txtar

@@ -0,0 +1,46 @@
+# Test that only packages explicitly implied from command line
+# arguments and their dependencies are scanned.
+# See https://cuelang.org/issue/3155 for context.
+
+# TODO: this should not fail.
+! exec cue eval ./a
+
+cmp stderr stderr.golden
+-- stderr.golden --
+cannot enumerate all module imports: cannot read "c/invalid.cue": expected 'STRING', found 'INT' 435
+-- cue.mod/module.cue --
+module: "mod.com"
+language: version: "v0.9.0"
+deps: "example.com/foo": v: "v0.0.1"
+-- root.cue --
+package root
+
+import "mod.com/x"
+
+x
+-- a/a.cue --
+package a
+
+import "mod.com/b"
+
+b
+-- b/b.cue --
+package b
+import "example.com/foo"
+b: 5
+foo
+-- c/invalid.cue --
+// This file should be ignored because it's not part of the dependencies
+// mentioned on the command line.
+// We use invalid syntax to surface an error if the file is not ignored as expected.
+package c
+import 435
+
+-- _registry/example.com_foo_v0.0.1/cue.mod/module.cue --
+module: "example.com/foo@v0"
+language: version: "v0.8.0"
+
+-- _registry/example.com_foo_v0.0.1/foo.cue --
+package foo
+foo: true
+