tool/listpkgs: add --has-go-generate filter flag too

For use in parallelizing go:generate up-to-date checks.

Updates tailscale/corp#28679

Change-Id: Ifc31c56de4225ba2e0fc048b0f18974dc2f2fc82
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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