Change `src` to `apps`

1. Removed `setup_eunit` plugin
2. Modified rebar.config.script to use correct path
3. `fauxton` is a Non-Erlang deps that rebar3 cannot fetch, so use
`configure` to download

1. Use `couch/rebar.config.script` to get dependencies to avoid
dependency cycle errors.
2. Modify Makefile to pass `make all`

1. Modify `couchdb-build-releases.sh` to copy rebar3
2. Modify the path to docs

1. rebar3 doesn't support `[raw]`
2. Mimic fauxton as an OTP application
See apache/couchdb-fauxton@2aa7845

Create a rebar3 ic setup_eunit plugin to keep the same functionality
as the previous one. Since we need to use `setup_eunit.template`, so
move it to `plugins/ic/priv` folder.

Commands:
`rebar3 help ic`, `rebar3 help ic setup_eunit`,
`rebar3 ic`, `rebar3 ic setup_eunit [-f]`, `rebar3 ic setup_eunit -f`,
and `make setup-eunit`

1. Removed `-r` and `skip_apps` since rebar3 does not have them.
2. Copy `config`, `b64url`, `jiffy`, etc. to `apps` folder to avoid
`application not exist in the project` error.
3. Modified paths since rebar and rebar3 used different paths.

Commands: `make eunit`, `make eunit apps=config`

`./dev/run` is using `reltoo.config`
`rebar3 release` is using `relx`

For `make release`, changed `couchdb` with `start` because it's hardcoded
in relx. see https://github.com/erlware/relx/blob/main/src/rlx_assemble.erl#L828