Fix a build issue when `go mod vendor` was used to vendor all the sources.
This now includes Mage (`github.com/magefile/mage/mage`), so that the
build tool itself can be built too.
`internal/tools/tools.go` is a bit of a hacky workaround a limitation of
`go mod tidy`. It will never be built, but `go mod tidy` will see the
packages imported here as dependencies of the Flamenco project, and not
remove them from `go.mod`.
This is meant for build-time requirements that are otherwise never
imported as Go libraries, like our OpenAPI code generator.