internal/frontend: redirect to details page if search exact path is found
internal/frontend: redirect to details page if search exact path is found

When searching on godoc.org, if an exact path is found, the user will be
redirected to the package page (example:
https://godoc.org/?q=golang.org/x/tools redirects to
https://godoc.org/golang.org/x/tools).

This behavior is now replicated on the discovery site.

Fixes b/131859851

Change-Id: Ie3e51f1857ba88110dcc7e1f7524abbbb3ae5951
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/460701
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

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