cue/load: avoid "user:" and "instance:" error prefixes

The `cue/load` package currently creates errors with `user:` and/or
`instance:` prefixes, which might make sense in the context of
the Go API, but don't really make sense on the command line,
as evidenced by comments like this:

https://review.gerrithub.io/c/cue-lang/cue/+/1194664/8/cmd/cue/cmd/testdata/script/issue174.txtar

This change removes those prefixes.

This doesn't appear to be covered by any tests, although
these prefixes do show up in some errors printed by a later
CL, which this improves.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I66670e94c4bd0a45d84a2ed4d6dec6de8d9db758
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194762
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

cue/load/config.go

@@ -469,7 +469,7 @@ func (c *Config) newErrInstance(err error) *build.Instance {
 	i := c.Context.NewInstance("", nil)
 	i.Root = c.ModuleRoot
 	i.Module = c.Module
-	i.Err = errors.Promote(err, "instance")
+	i.Err = errors.Promote(err, "")
 	return i
 }
 

cue/load/loader.go

@@ -143,10 +143,7 @@ func (l *loader) cueFilesPackage(files []*build.File) *build.Instance {
 	pkg.User = true
 	l.addFiles(l.cfg.Dir, pkg)
 
-	l.stk.Push("user")
 	_ = pkg.Complete()
-	l.stk.Pop()
-	//pkg.LocalPrefix = dirToImportPath(dir)
 	pkg.DisplayPath = "command-line-arguments"
 
 	return pkg