Go: x/pkgsite: split postgres.UpsertVersionState into InsertModuleVersionState and UpdateModuleVersionState

[julieqiu]

At the moment, postgres.UpsertModuleVersionState is overloaded: it is used both to insert new versions from the index, and update versions after a fetch. Refactor the following:

• Split UpsertModuleVersionState into InsertModuleVersionState and UpdateModuleVersionState. Notably:
  
  
  
  • InsertModuleVersionState does not accept a code or fetch error.
  
  
  • UpdateModuleVersionState does not accept an index timestamp.
  
  
• On fetch, call UpdateModuleVersionState and simply log NotFound errors, which would occur if the version did not come from the index

This would result in some duplicate processing, because if we fetch a version that is not in the index through the proxy, it will subsequently be added to the index. However, this would be easier to reason about: the module_version_state table should have a 1:1 correspondence with index.golang.org, and timestamps should all be correct.

Related: #39621

[AndersonQ]

On it :)