cloud/autocert: don't import autocert; use names from cloud/https
cloud/autocert: don't import autocert; use names from cloud/https

The upspin.io repository vendors the golang.org/x/crypto/acme/autocert
package, so when the autocert package in this repository imported the
autocert package from x/crypto, it was seeing a different package.
By using these types exported from the cloud/https package, the sentinel
error value ErrAutocertCacheMiss will be equivalent to the one that the
vendored x/crypto/acme/autocert package expects. Without this change,
renewing of Let's Encrypt certs was silently broken.

This workaround will go away once we have resolved issue upspin/upspin#214.

Change-Id: Iccc831a0cdf80d5a310fa278ea9b518daf669f5d
Reviewed-on: https://upspin-review.googlesource.com/11960
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/gcp,upspin/gcp