x/pkgsite: prefer prerelease over pseudo when defining @latest

[julieqiu]

go help goproxy says:

When resolving the latest version of a module, the go command will request
/@v/list, then, if no suitable versions are found, <module>/@latest.
The go command prefers, in order: the semantically highest release version,
the semantically highest pre-release version, and the chronologically
most recent pseudo-version. In Go 1.12 and earlier, the go command considered
pseudo-versions in /@v/list to be pre-release versions, but this is
no longer true since Go 1.13.

We currently implement the older meaning of latest (highest release, then highest anything else).
We should change that to prefer pre-release versions to pseudo-versions.

Example: pkg.go.dev/github.com/Netflix/titus-api-definitions shows
the pseudo-version v0.0.1-rc9.0.20200212222136-a4a89636720b

but go get github.com/Netflix/titus-api-definitions@latest fetches
v0.0.01-rc9 and https://proxy.golang.org/github.com/!netflix/titus-api-definitions/@latest:

{"Version":"v0.0.1-rc9","Time":"2017-10-31T18:43:32Z"}

[gopherbot]

Change https://golang.org/cl/254577 mentions this issue: internal/postgres: prefer prerelease over pseudo as latest version

[julieqiu]

Fixed.