-validate should not use HEAD requests

[axw]

Per https://go.dev/ref/mod#go-get, the HTTP GET method should be used with ?go-get=1 for identifying the VCS location for a vanity domain. -validate is issuing a HEAD request, which has recently started failing for go.uber.org modules.

[lahsivjar]

2023/08/28 13:11:15 ERROR: Invalid URL [https://go.uber.org/zap] for dependency [go.uber.org/zap]: request returned status code 404
2023/08/28 13:11:18 ERROR: Invalid URL [https://gonum.org/v1/gonum] for dependency [gonum.org/v1/gonum]: request returned status code 404
2023/08/28 13:11:18 ERROR: Invalid URL [https://go.uber.org/multierr] for dependency [go.uber.org/multierr]: request returned status code 404
2023/08/28 13:11:22 ERROR: Invalid URL [https://howett.net/plist] for dependency [howett.net/plist]: request returned status code 404

Some more failing modules.