internal/proxydatasource: continue searching for module on HTTP 410
internal/proxydatasource: continue searching for module on HTTP 410

The proxy returns 410, rather than 404, when a module path is not found.

Change-Id: I2fed459cb18373ed16c2fc7869886bcf62f4543c
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/544157
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite