cmd/compile: bad "cannot declare name" error after dot import

[mdempsky]

This package is valid, but cmd/compile rejects it with "cannot declare name bytes.Buffer":

package p

import . "bytes"

var _ Buffer // used import "bytes"

func f() {
        Buffer := 0
        _ = Buffer
}

[mdempsky]

This is similar to #22072, but likely easier to fix.

The root problem is when we have an assignment like x := y we're noding the left-hand side (x) like it's a general Go expression (using exprList), but then later we apply colasdefn to re-declare identifiers as appropriate.

Instead noder should probably apply colasdefn's logic directly to the package syntax AST.

[gopherbot]

Change https://golang.org/cl/66810 mentions this issue: cmd/compile: allow := to shadow dot-imported names