This detects existence of Hex registry at
$HOME/.cache/rebar3/hex/default/registry and skips "rebar3 update" step.
It also detect presence of bootstrap dependencies in _build/default/lib/
and skips fetching them.

fix #790

Removed separate compilers
Resolves apps to build
Finds avail deps before pulling/building
Includes relx
Simplifies build commands

If you happen to fetch a zip archive of the git repo and try to build
from that, you may, for example, ask erlc to build src/._rebar.erl.
._* are OS X resource forks and not real .erl files. This may also
happen with network filesystems on OS X. To fix that, limit the
files compiled by rebar to include only those which start with
a letter or a digit.

* fix overlong lines
* where appropriate use %% instead of %

If you invoke 'make debug', then bootstrap executes rebar:main/1 with
the extra argument "debug" and rebar treats that as a command. There is
no such command.

Without this patch you will see the following on bootstrap:
Command 'debug' not understood or not applicable

* re-compile rebar.erl instead of rebar_core to define VCS_INFO
* append "-dirty" if vcs indicates unclean repo status