govulncheck: adjust support to handle relative paths

[zpavlinovic]

As of v1.1.0, govulncheck JSON paths are relative to their enclosing module, rather than being absolute as earlier. This change was done to increase portability and to not leak information about the user filesystem.

If vscode-go starts using [email protected] or newer, the chances are the relative paths will break things. However, each path is relative to their enclosing module (including stdlib) and the module information is available in JSON. This should be sufficient to compute absolute paths on the local machine.