x/pkgsite: latest version should not be +incompatible

[jba]

When deciding which version of a module is the latest, if there is a version with a go.mod file, prefer it to incompatible versions.

Example: given versions

• v2.0.0+incompatible
• v1.2.3
  we would display the first as latest. We should display the second.

[jba]

The go command behaves similarly as of 1.14: https://golang.org/doc/go1.14#incompatible-versions.

[julieqiu]

/cc @fflewddur for context

[zot]

Nice! Looks like this would resolve the long-standing issue with libp2p docs: libp2p/go-libp2p#805