Update filepaths and go.mod contents to support default release tags

[ZachtimusPrime]

• move go.mod to root
• move ./splunk/v2 to ./v2
• update go.mod to reflect the change of path for the module so that specific release tags don't have to be created in the format splunk/vX.X.X

[antoineco]

One note: after that change, it will be necessary to alias the package import for the package name to be a valid Go import (e.g. "splunk").

// before
import "github.com/ZachtimusPrime/Go-Splunk-HTTP/splunk/v2"

func f() {
    // implicitly available as "splunk"
    _ = &splunk.Client{}
}

// after
import splunk "github.com/ZachtimusPrime/Go-Splunk-HTTP/v2"

func f() {
    _ = &splunk.Client{}
}

I'm fine with that, but it's worth documenting it in the README.