cmd/compile, importers: implement support for Alias types nodes in export data format

[griesemer]

This is a follow-up issue on #63223 to track the support for Alias types in importers.
@mdempsky
@findleyr

[griesemer]

cc @mvdan

[gopherbot]

This issue is currently labeled as early-in-cycle for Go 1.23.
That time is now, so a friendly reminder to look at it again.

[mvdan]

Just checking in - is this still planned for Go 1.23? Once it lands in master, I'm more than happy to give it a try :)

[griesemer]

Yes. We're just super busy at the moment. Thanks.