Vendor structure

[ferd]

This PR is in two parts, and is a proposal to switch up how we do things to facilitate
corporate use cases and the job of linux distribution package maintainers.

Migrate to an umbrella structure

This is a prerequisite for vendoring and demands that we move all source
files around, and update the helper scripts (like bootstrap) to work
with it.

This should have no consequence on rebar3 users aside from needing to
fetch a git_subdir if they need the content for it.

Vendor all dependencies, make bootstrap use them

This makes rebar3 use de-facto offline bootstrap, and avoids hitting the network
to fetch certs anywhere else but when a developer flushes all source files and
re-fetches certifi.

This is done by calling rebar3 as experimental vendor.

[ferd]

Looks like this is blowing up shelltests, mostly around releases. Not too sure about that, will have to dig a bit.

E: was related to missing template data in a partially moved escript config

[ferd]

Oh, fun gotcha: this breaks dialyzer by analyzing deps that were previously not covered...

[zuiderkwast]

Will you release this in a minor version soon?

[ferd]

Probably could be doable, we haven't heard any feedback positive or otherwise though, so it's hard to say if it's ready to go.

[zuiderkwast]

We have tested it and it seems to work well. :-) We would prefer a release.

[zuiderkwast]

Still interested in a release. Thanks.

[ferd]

It's finally out, needed CI to work end-to-end to make it all go!