SidebarGroup does not use @theme alias

[florevallatmrf]

• I confirm that this is an issue rather than a question.

Bug report

Steps to reproduce

• Using the default theme
• Shadow the SidebarLinks component

What is expected?

All instances of SidebarLinks in the app should use the shadowed version

What is actually happening?

Only first level groups are using the shadowed version, because SidebarGroup is not using the @theme alias when importing the component:
https://github.com/vuejs/vuepress/blob/master/packages/@vuepress/theme-default/components/SidebarGroup.vue#L66