go.mod should have /v2 at the end of the module name

[andig]

This is a follow-up of #19 which still appears broken. Consider this:

❯ go mod init foo
go: creating new go.mod: module foo

❯ go get github.com/influxdata/go-syslog@latest && cat go.mod 
module foo

go 1.13

require github.com/influxdata/go-syslog v1.0.1 // indirect

❯ go get github.com/influxdata/go-syslog@v2.0.0
go: finding github.com/influxdata/go-syslog v2.0.0
go get github.com/influxdata/go-syslog@v2.0.0: github.com/influxdata/go-syslog@v2.0.0: invalid version: module contains a go.mod file, so major version must be compatible: should be v0 or v1, not v2

go get will never install a v2 version as the import path does not match. See https://github.com/golang/go/wiki/Modules#semantic-import-versioning for path names.

My feeling is that as this stage the right thing would be to release a 3.0 version (branched from 2.0) with a /v3 import path.

[andig]

Right now this prevents porting telegraf to go modules (influxdata/telegraf#6118) as v1 doesn't have the required structure and v2 cannot be imported:

❯ go run cmd/telegraf/telegraf.go -debug -sample-config
build command-line-arguments: cannot load github.com/influxdata/go-syslog: module github.com/influxdata/go-syslog@latest found (v1.0.1), but does not contain package github.com/influxdata/go-syslog

[andig]

/cc @leodido @elliotmr

[andig]

Looks like 6b97c07 basically fixed the wrong thing. It should have worked before that commit.