Add CI checks to ensure go.mod has no directives such as `replace` or `exclude`

[0pcom]

Replace and exclude directives in go.mod must be forbidden due to the following type of error

$ go run github.com/skycoin/skywire@develop
go: github.com/skycoin/skywire@develop (in github.com/skycoin/[email protected]):
	The go.mod file for the module providing named packages contains one or
	more exclude directives. It must not contain directives that would cause
	it to be interpreted differently than if it were the main module.

I run every skywire utility @develop - so this breaks things for me in a major way if forbidden directives are added to go.mod.

It's fine to use these directives locally for testing but they must never be committed.