Fix module path and restore cmd/ omitted by gitignore

Rename module github.com/johnellis/stalwart-migrator ->
github.com/LINUXexpert-org/stalwart-migrator to match the repository
location, so the module resolves under `go get`.

The initial commit's .gitignore listed the compiled binary as a bare
`stalwart-migrate` pattern, which Git matches at any depth -- so it also
excluded the cmd/stalwart-migrate/ source directory, and the initial
commit shipped without the CLI entrypoint. Anchor the pattern to the
repo root as /stalwart-migrate and add the four missing files.

go build, go vet, and go test ./... all pass.

internal/recovery/recovery_test.go

@@ -8,7 +8,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/johnellis/stalwart-migrator/internal/checkpoint"
+	"github.com/LINUXexpert-org/stalwart-migrator/internal/checkpoint"
 )
 
 func TestRecoveryRunEndToEndAndResume(t *testing.T) {