lib: Move to go modules with regret

[purpleidea]

I'm incredibly annoyed that go modules are being forced onto us, as the
old system with vendor/ and git submodules worked great. Unfortunately,
so much FUD around git submodules, even by well-known, highly paid
engineers has caused this solution to be deemed most unacceptable.

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

Thanks to frebib for his help with this suffering.

Tips:

• please read the style guide before submitting your patch:
  docs/style-guide.md

• commit message titles must be in the form:

topic: Capitalized message with no trailing period

or:

topic, topic2: Capitalized message with no trailing period

• golang code must be formatted according to the standard, please run:

make gofmt		# formats the entire project correctly

or format a single golang file correctly:

gofmt -w yourcode.go

• please rebase your patch against current git master:

git checkout master
git pull origin master
git checkout your-feature
git rebase master
git push your-remote your-feature
hub pull-request	# or submit with the github web ui

• after a patch review, please ping @purpleidea so we know to re-review:

# make changes based on reviews...
git add -p		# add new changes
git commit --amend	# combine with existing commit
git push your-remote your-feature -f
# now ping @purpleidea in the github PR since it doesn't notify us automatically

Thanks for contributing to mgmt and welcome to the team!

[purpleidea]

@frebib

[frebib]

https://github.com/frebib/mgmt/commits/feat/go-mod-fixes
Here are my fixes. Please look over the last commit as I removed anything that seemed no longer applicable. Feel free to exclude any of my removals as you see fit, then hopefully we can get this merged

[frebib]

You're also going to want to remove the rest of .gitmodules because any partial vendor directory will (probably) confuse go-mod. It'll either be ignored outright or cause build failures depending on how you invoke go build -mod=???
Updated my branch

[purpleidea]

Nice, you rebased on top. Fantastic. Will look later tonight.

[frebib]

I found that with go modules, gometalinter fails to install, for some reason. You may need this commit to make the tests pass
frebib@8fde6aa

[frebib]

We need #670 finished and merged. It would have picked this up!

main.go:52: File is not `goimports`-ed (goimports)
		Flags: flags,

diff --git a/main.go b/main.go
index 8eba6fc..8c36be1 100644
--- a/main.go
+++ b/main.go
@@ -49,7 +49,7 @@ func main() {
                Program: program,
                Version: version,
                Copying: copying,
-               Flags: flags,
+               Flags:   flags,
        }
        if err := mgmt.CLI(cliArgs); err != nil {
                fmt.Println(err)

[purpleidea]

Replaced by #674