lib: Move to go modules and shed a few tears

The old system with vendor/ and git submodules worked great,
unfortunately FUD around git submodules seemed to scare people away and
golang moved to a go.mod system that adds a new lock file format instead
of using the built-in git version. It's now almost impossible to use
modern golang without this, so we've switched.

So much for the golang compatibility promise-- turns out it doesn't
apply to the useful parts that I actually care about like this.

Thanks to frebib for his incredibly valuable contributions to this
patch. This snide commit message is mine alone.

This patch also mixes in some changes due to legacy golang as we've also
bumped the minimum version to 1.16 in the docs and tests.

Lastly, we had to disable some tests and fix up a few other misc things
to get this passing. We've definitely hot bugs in the go.mod system, and
our Makefile tries to workaround those.

go.mod

@@ -0,0 +1,57 @@
+module github.com/purpleidea/mgmt
+
+go 1.16
+
+require (
+	cloud.google.com/go v0.54.0 // indirect
+	github.com/Microsoft/go-winio v0.4.17 // indirect
+	github.com/aws/aws-sdk-go v1.40.49
+	github.com/containerd/containerd v1.4.9 // indirect
+	github.com/coredhcp/coredhcp v0.0.0-20210830115404-2176f33418f4
+	github.com/coreos/go-systemd/v22 v22.3.2
+	github.com/cyphar/filepath-securejoin v0.2.3
+	github.com/davecgh/go-spew v1.1.1
+	github.com/deniswernert/go-fstab v0.0.0-20141204152952-eb4090f26517
+	github.com/docker/distribution v2.7.1+incompatible // indirect
+	github.com/docker/docker v20.10.8+incompatible
+	github.com/docker/go-connections v0.4.0
+	github.com/docker/go-units v0.4.0 // indirect
+	github.com/fsnotify/fsnotify v1.5.1
+	github.com/godbus/dbus/v5 v5.0.4
+	github.com/google/uuid v1.2.0 // indirect
+	github.com/gorilla/mux v1.7.2 // indirect
+	github.com/hashicorp/consul/api v1.1.0
+	github.com/hashicorp/go-multierror v1.0.0
+	github.com/hashicorp/hil v0.0.0-20210521165536-27a72121fd40
+	github.com/iancoleman/strcase v0.2.0
+	github.com/insomniacslk/dhcp v0.0.0-20210827173440-b95caade3eac
+	github.com/kr/pretty v0.2.1 // indirect
+	github.com/kylelemons/godebug v1.1.0
+	github.com/libvirt/libvirt-go v7.4.0+incompatible
+	github.com/libvirt/libvirt-go-xml v7.4.0+incompatible
+	github.com/matttproud/golang_protobuf_extensions v1.0.2-0.20181231171920-c182affec369 // indirect
+	github.com/moby/term v0.0.0-20200312100748-672ec06f55cd // indirect
+	github.com/morikuni/aec v1.0.0 // indirect
+	github.com/opencontainers/go-digest v1.0.0 // indirect
+	github.com/opencontainers/image-spec v1.0.1 // indirect
+	github.com/pborman/uuid v1.2.1
+	github.com/pin/tftp v0.0.0-20210809155059-0161c5dd2e96
+	github.com/pkg/errors v0.9.1
+	github.com/prometheus/client_golang v1.11.0
+	github.com/sanity-io/litter v1.5.1
+	github.com/sirupsen/logrus v1.8.1 // indirect
+	github.com/spf13/afero v1.5.1
+	github.com/urfave/cli/v2 v2.3.0
+	github.com/vishvananda/netlink v1.1.1-0.20201029203352-d40f9887b852
+	go.etcd.io/etcd/api/v3 v3.5.0
+	go.etcd.io/etcd/client/pkg/v3 v3.5.0
+	go.etcd.io/etcd/client/v3 v3.5.0
+	go.etcd.io/etcd/server/v3 v3.5.0
+	golang.org/x/crypto v0.0.0-20210322153248-0c34fe9e7dc2
+	golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c
+	golang.org/x/time v0.0.0-20210220033141-f8bda1e9f3ba
+	gopkg.in/src-d/go-git.v4 v4.13.1
+	gopkg.in/yaml.v2 v2.4.0
+	gotest.tools/v3 v3.0.3 // indirect
+	honnef.co/go/augeas v0.0.0-20161110001225-ca62e35ed6b8
+)