Prettify all of common test output.

This uses cth_readable to:
- silence error_logger output to the shell unless a test fails
- silence ct:pal output to the shell unless a test fails

I have currently not baked in any way to disable this behaviour, but I
figured if it is required, there is time to do it before the final
3.0.0 release.

fix typo of guarantee. h/t evan

do not sort mib_first_files during merge_opts, order must be kept

upgrade relx to 3.8.0, fixes bug in upgrade_install escript

calculate coverage info as late as possible in ct provider execution

fixes for cover under new compiler/eunit/ct

rebar_prv_common_test refactor

* remove partial support for ct `test_spec` until it can be done properly
  and warn if `test_spec` is present in test opts
* use new compiler functionality to reduce complexity of provider
* reduce command line options available to those that can be
  supported properly

fix #903: use app discover of entire plugins dir when listing

Rebuild PLT when beams no longer exist

add an option to soft purge rather than purge old code

* only try to cover compile directories that actually exist
* recover from failures where source files don't contain required
  attributes for cover compilation and print warning

at the cost of some SASL warnings this prevents rebar3 from terminating
processes when reloading their code before running tests

delay validation of eunit tests until just before running

prevent rebar3 from reloading it's own path and terminating itself

prevents code reloading by temporary marking of rebar3's ebin dir
as sticky during code refresh

move test profile bootstrap into core

dedup `src_dirs` and `extra_src_dirs` on read

only clean project apps once even when doing --all

compiler refactor/eunit & ct fixes

* modify compiler interface to work on either application objects or
  directories containing source files
* compile all sources in `src_dirs` to the application `ebin` dir and
  all sources in `extra_src_dirs` to a directory mirroring it's
  position in the app's `_build` directory. for example, `apps/foo/more`
  would compile to `_build/default/lib/foo/more`
  for `extra_src_dirs` in the root of a project with multiple
  applications (so orphan directories that don't "belong" to an
  application) compile to `_build/default/extras/more`
* copy directories specified in `extra_src_dirs` into the `_build`
  directory so tools like `ct` and `xref` that expect source to be
  in a particular location still work
* clean compiled artifacts from all `extra_src_dirs`
* alter `eunit`, `ct` and `cover` to work with the new directory
  structure
* billions of new tests

This reverts commit 9d82215296.

return more useful error message (failed to download) when pkg http req fails