cmd/cue: use default module name for cue mod init

Currently if no arguments are passed to `cue mod init`, it creates
a module file with an empty path, which isn't valid.

This changes it so it uses an arbitrary module path (`cue.example`)
instead.

Fixes #3260.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I4388d49ee6ff208cd54ec2bdee724cbd8b6f0ebd
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1198143
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: rogpeppe

cmd/cue/cmd/modinit.go

@@ -34,9 +34,8 @@ creating a new module rooted at the current directory. The cue.mod
 directory must not already exist. A legacy cue.mod file in the current
 directory is moved to the new subdirectory.
 
-A module name is optional, but if it is not given, a package
-within the module cannot import another package defined
-in the module.
+If the module name is not provided, a default module path (cue.example) will be
+used.
 `,
 		RunE: mkRunE(c, runModInit),
 	}
@@ -48,7 +47,7 @@ in the module.
 }
 
 func runModInit(cmd *Command, args []string) (err error) {
-	modulePath := ""
+	modulePath := "cue.example"
 	if len(args) > 0 {
 		if len(args) != 1 {
 			return fmt.Errorf("too many arguments")

cmd/cue/cmd/testdata/script/modinit_noargs.txtar

@@ -7,7 +7,7 @@ exists cue.mod/usr
 exists cue.mod/pkg
 
 -- want-module.cue --
-module: ""
+module: "cue.example"
 language: {
 	version: "$CUE_LANGUAGE_VERSION"
 }