cmd/go/internal/modfetch: skip symlinks in (*coderepo).Zip

Tested manually.

Before:

	$ go mod init golang.org/issue/scratch
	go: creating new go.mod: module golang.org/issue/scratch
	$ go1.11.2 mod download github.com/rogpeppe/test2@latest
	go: finding github.com/rogpeppe/test2 v0.0.11
	$ find $GOPATH -name goodbye
	/tmp/tmp.Y8a8UzX3zD/_gopath/pkg/mod/github.com/rogpeppe/test2@v0.0.11/tests/goodbye
	$ cat $(find $GOPATH -name goodbye)
	hello

After:

	$ go mod init golang.org/issue/scratch
	go: creating new go.mod: module golang.org/issue/scratch
	$ go mod download github.com/rogpeppe/test2@latest
	go: finding github.com/rogpeppe/test2 v0.0.11
	$ find $GOPATH -name goodbye
	$ find $GOPATH -name hello
	/tmp/tmp.Zo0jhfLaRs/_gopath/pkg/mod/github.com/rogpeppe/test2@v0.0.11/tests/hello

A proper regression test would require one of:
• a new entry in the vcs-test server (feasible but tedious, and not easily updated by open-source contributors), or
• a way to set up an HTTPS proxy in a script_test, or
• a way to explicitly populate the module cache from the contents of a local repository (#28835).

Fixes #27093
Updates #28835

Change-Id: I72702a7e791f8815965f0f87c82a30df4d6f0151
Reviewed-on: https://go-review.googlesource.com/c/153819
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modfetch/coderepo.go

@@ -489,6 +489,11 @@ func (r *codeRepo) Zip(dst io.Writer, version string) error {
 	}
 
 	for _, zf := range zr.File {
+		if !zf.FileInfo().Mode().IsRegular() {
+			// Skip symlinks (golang.org/issue/27093).
+			continue
+		}
+
 		if topPrefix == "" {
 			i := strings.Index(zf.Name, "/")
 			if i < 0 {