cmd/go: fix 'go get vanity/repo/...' in clean GOPATH

The pattern was only working if the checkout had
already been done, but the code was trying to make
it work even the first time. Test and fix.

Fixes #8335.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews, iant
https://golang.org/cl/146310043

src/cmd/go/test.bash

@@ -948,6 +948,18 @@ elif ! grep "$GOARCH test3.go p xyzp/test3.go/123" testdata/std.out > /dev/null;
 	ok=false
 fi
 
+TEST go get works with vanity wildcards
+d=$(mktemp -d -t testgoXXX)
+export GOPATH=$d
+if ! ./testgo get -u rsc.io/pdf/...; then
+	ok=false
+elif [ ! -x $d/bin/pdfpasswd ]; then
+	echo did not build rsc.io/pdf/pdfpasswd
+	ok=false
+fi
+unset GOPATH
+rm -rf $d
+
 # clean up
 if $started; then stop; fi
 rm -rf testdata/bin testdata/bin1

src/cmd/go/vcs.go

@@ -361,7 +361,14 @@ var httpPrefixRE = regexp.MustCompile(`^https?:`)
 func repoRootForImportPath(importPath string) (*repoRoot, error) {
 	rr, err := repoRootForImportPathStatic(importPath, "")
 	if err == errUnknownSite {
-		rr, err = repoRootForImportDynamic(importPath)
+		// If there are wildcards, look up the thing before the wildcard,
+		// hoping it applies to the wildcarded parts too.
+		// This makes 'go get rsc.io/pdf/...' work in a fresh GOPATH.
+		lookup := strings.TrimSuffix(importPath, "/...")
+		if i := strings.Index(lookup, "/.../"); i >= 0 {
+			lookup = lookup[:i]
+		}
+		rr, err = repoRootForImportDynamic(lookup)
 
 		// repoRootForImportDynamic returns error detail
 		// that is irrelevant if the user didn't intend to use a