cmd/go/internal/modfetch: skip symlinks in (*coderepo).Zip
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 <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>

cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,josharian/go,akutz/go,CAFxX/go,nawawi/go,golang/go,josharian/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,akutz/go,akutz/go,akutz/go,josharian/go,golang/go,golang/go,CAFxX/go,cloudflare/go,akutz/go,nawawi/go,CAFxX/go,josharian/go,CAFxX/go,golang/go,akutz/go,josharian/go,CAFxX/go,nawawi/go,josharian/go,golang/go,josharian/go,akutz/go,golang/go,josharian/go,CAFxX/go,nawawi/go,golang/go,akutz/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go