x/pkgsite: add a function to get the version history for a path

[julieqiu]

As part of the new data model migration (#39629), we will be deprecating these functions:

• LegacyGetPsuedoVersionsForModule
• LegacyGetPsuedoVersionsForPackageSeries
• LegacyGetTaggedVersionsForModule
• LegacyGetTaggedVersionsForPackageSeries

See http://github.com/golang/pkgsite/blob/master/internal/postgres/version.go.

We want fetch version history from the paths table instead.

The query to execute is:

SELECT
    p.path,
    p.module_path,
    p.version,
    p.commit_time
FROM modules m
INNER JOIN paths p
ON p.module_id = m.id
WHERE
    m.module_path = $1
    AND m.version = $2
    AND p.path = $3
ORDER BY
   CASE WHEN version_type = 'release' THEN 0
              WHEN version_type = 'prerelease' THEN 1
              ELSE 2
              END,
   m.sort_version DESC;

If we know about release or pre-release versions for path, return them all. Don't return an pseudoversions.

If we only know about pseudoversions, return the 10 most recent.

[julieqiu]

Implemented in https://golang.org/cl/242881.