undo CL 87300043 / 1dc800571456

This breaks "go get -d repo/path/...".

««« original CL description
cmd/go: do not miss an error if import path contains "cmd/something"

Fixes #7638

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/87300043
»»»

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/87890043

src/cmd/go/get.go

@@ -143,10 +143,6 @@ var downloadRootCache = map[string]bool{}
 // for the package named by the argument.
 func download(arg string, stk *importStack, getTestDeps bool) {
 	p := loadPackage(arg, stk)
-	if p.Error != nil {
-		errorf("%s", p.Error)
-		return
-	}
 
 	// There's nothing to do if this is a package in the standard library.
 	if p.Standard {