x/pkgsite: module latest-version information

[jba]

Pkgsite needs to track three kinds of latest versions for each module:

• The raw latest version (see x/pkgsite: handle raw latest versions #44437)—determines retractions and deprecations;
• The cooked latest version, which is the raw one after applying retractions—what go get will download;
• The good latest version, which is the latest version that pkg.go.dev has useful information for.

We will have a single table, module_latest_versions, to track all of these. The table will be updated by the worker each time a module (at any version) is processed. It will be consulted by GetUnitMeta and other DataSource methods that require the latest version of a module.

Usually all three kinds of latest version will be the same, but sometimes they will differ. Pkgsite will always show the good latest version when the URL is unversioned (that is, it does not contain "@version"). If the cooked latest version differs, the page will say so.

[gopherbot]

Change https://golang.org/cl/297649 mentions this issue: internal/version: generalize Later to pseudo-versions

[gopherbot]

Change https://golang.org/cl/297689 mentions this issue: migrations: add latest_module_versions table

[gopherbot]

Change https://golang.org/cl/297849 mentions this issue: internal: add LatestModuleVersions

[gopherbot]

Change https://golang.org/cl/297889 mentions this issue: internal/version: implement full latest logic

[gopherbot]

Change https://golang.org/cl/298229 mentions this issue: internal/postgres: get/update LatestModuleVersions

[gopherbot]

Change https://golang.org/cl/298230 mentions this issue: internal/fetch: compute latest module versions from proxy