cmd/go: diagnose non-canonical import paths before compilation

cmd/go: diagnose non-canonical import paths before compilation

If we leave it for compilation sometimes the error appears first
in derived vendor paths, without any indication where they came from.
This is better.

$ go1.7 build canonical/d
cmd/go/testdata/src/canonical/a/a.go:3: non-canonical import path "canonical/a//vendor/c" (should be "canonical/a/vendor/c")
cmd/go/testdata/src/canonical/a/a.go:3: can't find import: "canonical/a//vendor/c"

$ go build canonical/d
package canonical/d
	imports canonical/b
	imports canonical/a/: non-canonical import path: "canonical/a/" should be "canonical/a"
$

Fixes #16954.

Change-Id: I315ccec92a00d98a08c139b3dc4e17dbc640edd0
Reviewed-on: https://go-review.googlesource.com/31668
Reviewed-by: Quentin Smith <quentin@golang.org>