x/pkgsite: add GetReadme function

[julieqiu]

As part of the new data model migration (#39629), we need to add a function to fetch a README for a give path_id.

The function signature will be:

 GetReadme(ctx context.Context, pathID int) (_ *internal.Readme, err error)

The function will execute the query below:

SELECT
   file_path,
   contents
FROM
  readmes 
WHERE
  path_id = $1

[gopherbot]

Change https://golang.org/cl/250540 mentions this issue: internal/postgres: split GetDirectory into multiple functions