Use logrus without an alias

[na--]

In a lot of places in the k6 codebase, logrus is imported like this:

import (
	// ...
	log "github.com/sirupsen/logrus"
	// ...
)

As @mstoykov mentioned, importing it directly as logrus would likely be more sensible and idiomatic, to avoid confusion with the default Go stdlib log library and others.