Find libssh2 via pkg-config

We don't really need our own module to find libssh2. Using pkg-config
lets the standard tool do the work for us and let us fit more naturally
in the workflow as we respect the pkg-config search paths.

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I don't see a reason for us to have our own module. This was brought in with the original PR, but libssh2 provides its own pc file, so there's no reason for us to guess where the user has installed their library.