[dev.typealias] cmd/compile, go/importer: define export format and implement importing of type aliases

This defines the (tentative) export/import format for type aliases.

The compiler doesn't support type aliases yet, so while the code is present
it is guarded with a flag.

The export format for embedded (anonymous) fields now has three modes (mode 3 is new):
1) The original type name and the anonymous field name are the same, and the name is exported:
   we don't need the field name and write "" instead
2) The original type name and the anonymous field name are the same, and the name is not exported:
   we don't need the field name and write "?" instead, indicating that there is package info
3) The original type name and the anonymous field name are different:
   we do need the field name and write "@" followed by the field name (and possible package info)

For #18130.

Change-Id: I790dad826757233fa71396a210f966c6256b75d3
Reviewed-on: https://go-review.googlesource.com/35100Reviewed-by: Alan Donovan <adonovan@google.com>