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.