[dev.typeparams] cmd/compile/internal/types2: don't crash if import path is missing

In package syntax:
- fix parser appendGroup to not add nil entries
- non-string paths are syntax errors per the spec; report in parser
- document ImportDecl.Path invariants

In package types2:
- guard against absent paths

In package gc:
- guard against absent paths

Fixes #43190.

Change-Id: Ic6a06f6a96b7f519feaa1ceaf4376fc5ab0f0129
Reviewed-on: https://go-review.googlesource.com/c/go/+/278114


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>