cmd/compile: confusing error when using a keyword as package member

[mvdan]

$ go version
go version devel +447965d4e0 Sun Jan 27 04:18:10 2019 +0000 linux/amd64
$ cat f.go
package p

import "go/types"

func _() {
        var _ types.const
}
$ go build f.go
# command-line-arguments
./f.go:6:8: cannot refer to unexported name types._
./f.go:6:14: syntax error: unexpected const, expecting name

Of course, here I fat-fingered the type and meant types.Const. I encountered this in a real program, and the types._ error had me confused for a second. I presume that the first of the two errors should not be printed, as the second error contains all the information the user needs.

Interestingly enough, seems like the go/* packages don't do a particularly better job here:

f.go:6:14: expected 'IDENT', found 'const'
f.go:6:14: _ not declared by package types

[gopherbot]

Change https://golang.org/cl/160637 mentions this issue: cmd/compile: avoid confusing error on pkg.keyword

[mvdan]

Seemed easy enough, so I had a stab at this. I presume we should fix this in go/* as well, presumably go/parser.