Fixes #2096

Hello!

It's really nice to call rebar3 shell and have .config.src files automatically loaded, much like they are in Erlang releases. I was surprised to see that rebar3 ct doesn't do the same sort of substitution with environment variables. With these changes, it is now possible.

A big portion of this PR is moving consult_env_config/2 from rebar_priv_shell.erl to rebar_file_utils.erl.

I have some remaining questions/statements:

1. Would it be useful for rebar_file_utils:consult_config/2 read the extension of the file, and automatically call consult_env_config if the extension is .src? The branching for .config and .config.src is currently duplicated, but I'm not exactly sure if it's the right move to DRY in this case.
2. Please note, I changed the logic slightly in consult_config to only prepend RootDir if a config file consulted from a .config file is a relative path. erl -man config seems to recommend using absolute paths in sys.config files, so I wanted to be sure that we don't do any unfortunate path mangling when there an absolute path is found.
3. Should I go ahead and implement this functionality for eunit tests? The way forward appears to be to accept an env_file option in eunit_opts, add a .config.src file to test/rebar_eunit_SUITE_data/syscfg_app.zip, along with an appropriate env_file. I could probably submit that as another PR; just say the word :)
4. rebar_prv_common_test currently calls rebar_prv_shell:maybe_set_env_vars, introducing a dependency edge between two provider modules that didn't exist before. I don't mind moving maybe_set_env_vars to a module like rebar_file_utils if you'd like to avoid coupling the rebar_prv_shell and rebar_prv_common_test.