Do not parse go.mod files using regex

`taggit` [parses go.mod files using regex](https://github.com/modprox/taggit/blob/6aa061990baaef3bfde8611410575b2501c9d33f/internal/publish/module.go#L53). It should use a proper `go.mod` parser instead and switch to using the official package once it's published (there are plans to publish it as per [this comment](https://github.com/golang/go/issues/23966#issuecomment-377997161))