Fix various Dialyzer warnings

Some still remain from erl_type calls from Dialyzer, but most of them
are handled.

Decided to just ignore rebar_alias since playing with the type specs of
abstract code format is just a nightmare and hard to do cross-versions I
guess.

Handle Schema-less Proxy URLs in ENV vars

We can probably still move ahead without a cache if we must

We've had multiple tickets opened because of unclear PROXY settings when
the scheme is missing form the URI. To be helpful, we instead add them
dynamically whenever they're missing.

Example issues:
- https://github.com/erlang/rebar3/issues/1747
- https://github.com/erlang/rebar3/issues/1697

Make rebar3 work with dialyzer internally

We got funny interactions since PR #1656:

- the last `debug_info`-related option seen in a list of options after
  profile merge is kept, allowing later profiles from overtaking earlier
  ones
- if you go `rebar3 as a,b,c compile`, the options from profile A come
  before B, which come before C, so C's options win
- overrides are applied in order of profile as well, giving a priority
  to a later profile than an earlier one
- The values in overrides are prepended rather than suffixed to the
  existing list
- this means if we have to overrides adding options, such as `default`
  adding `no_debug_info', and `dialyze` adding `debug_info`, the
  results are `[debug_info]` as `dialyze` is applied first, and then
  `[no_debug_info, debug_info]` as `default` overrides are applied
- the final result is `no_debug_info` always winning when erl_opts are
  overriden specifically.

only `debug_info` options are going to suffer this, and in the context of
overrides. Other `erl_opts` should be fine. I'm not sure how that can be
fixed at all.

In the meanwhile, we can add support back while leaving the default to
not having debug information. This is done by:

- moving all `no_debug_info` options to the `prod` profile
- forcing `prod` to be called by `./bootstrap` so that most people keep
  getting no debug info
- anyone calling `rebar3 clean -a`  and then rebuilding with `rebar3
  escriptize` (i.e. rebar3 devs) get debug info going

This is up for review and discussion.

(#1741): Fix quotes in etag values

Bump cth_readable to 1.3.3

Fixes some lager warnings since config changes

Remove noisy debug message

It makes things worse than if it weren't there.
Fixes issue #1726

sort-as: force an order on multiple profiles

This reverts commit 0f7e6c31e9.

This reverts commit 4ad1db9733.

This reverts commit 3f8dd5eace.

use debug level for logs about bad package versioning

don't attempt retry of missing packages when updating index

Env for providers

rebar_package: do not return first package version for constraint with no match

Set unicode option for standard_io at entry point

fix hooks and plugins usage when cleaning build artifacts for deps