Internals: Switch to v2 go.mod, drop GOPATH, and fix Azure Pipelines (#595)

* Switched to v2 go.mod

Also set GO111MODULE=on in build stuff to always use Go modules
even when in GOPATH.

* Ensure go.mod, go.sum, and vendor are up to date

* Attempt to fix Azure pipelines

* Add set -e to properly fail on exit (it didn't seem to be
  propagating properly before).
* Set workingDirectory for GoFmt and GoGen (this might be why it
  fails unlike compile and unitests).

* Another attempt to fix Azure Pipelines

* Use the Go env template for all go-related jobs.

* Completely fixed Azure Pipelines

* Added a display name to GoFmt for consistency.
* Fixed diffs for GoFmt and GoGen.
* Show git status for checks.

* Drop GOPATH for tests

TODO: Do the same for integration tests.

* Drop GOPATH for integration tests

* Show more diffs

* Regenerate provider support matrix

This wasn't done in #590...

providers/namedotcom/nameservers.go

@@ -6,7 +6,7 @@ import (
 	"sort"
 	"strings"
 
-	"github.com/StackExchange/dnscontrol/models"
+	"github.com/StackExchange/dnscontrol/v2/models"
 	"github.com/namedotcom/go/namecom"
 )
 
@@ -57,7 +57,7 @@ func (n *NameCom) GetRegistrarCorrections(dc *models.DomainConfig) ([]*models.Co
 		expected = append(expected, ns.Name)
 		// FIXME(tlim): This should store a FQDN with no trailing ".".
 		// See pkg/nameservers/nameservers.go for details.
-		// Bug https://github.com/StackExchange/dnscontrol/issues/491
+		// Bug https://github.com/StackExchange/dnscontrol/v2/issues/491
 	}
 	sort.Strings(expected)
 	expectedNameservers := strings.Join(expected, ",")