Change module path up to first slash to include a dot (Go convention)

See https://go.dev/ref/mod#go-mod-file-ident:
```
The leading path element (up to the first slash, if any), by convention a domain name, must contain only lower-case ASCII letters, ASCII digits, dots (., U+002E), and dashes (-, U+002D); it must contain at least one dot and cannot start with a dash.
```