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...

build.ps1

@@ -10,11 +10,13 @@ if ($SHA -eq ""){
 
 $PKG = "github.com/StackExchange/dnscontrol"
 $DATE = [int][double]::Parse((Get-Date -UFormat %s))
-$FLAGS="-s -w -X main.SHA=$SHA -X main.BuildTime=$DATE"
+$FLAGS="-mod=readonly -s -w -X main.SHA=$SHA -X main.BuildTime=$DATE"
 Write-Host $FLAGS
 
 $OrigGOOS = $env:GOOS
 
+$env:GO111MODULE = "on"
+
 Write-Host 'Building Linux'
 $env:GOOS = "linux"
 go build -o dnscontrol-Linux -ldflags "$FLAGS" $PKG