Add documentation for {ref, <sha>} syntax rebar.config.sample file

Addresses: #130

Change mock_pkg_resource to use rebar_test_utils:create_empty_app so
that an ebin directory is created which is a good expectation for a
package. Also revert the check in rebar_fetch:download_source to check
for an ebin directory before adding it to the code path for package
installs. If a package does not have an ebin directory it is probably
a good for an exception to be thrown.

Fix a bug where packages are not added to the code path after
installation. Dependent applications that build from source are not
affected by this issue since the build_apps function in
rebar_prv_compiler takes care of the code path changes for them. It is
only the precompiled packages that suffer from this issue.

Add xref support for multi-app projects

Add xref support for multi-app projects. Instead of only adding the
top-level ebin directory to the xref server use
rebar_state:project_apps/1 to enumerate the applications for the
project and add the ebin directory for each one.

package dep names are binary, fix install deps and related tests

Add xref provider for cross reference analysis

* Add a provider for the xref tool for running cross reference
  analysis on a project. Most of the code has been ported directly
  from the rebar2 rebar_xref module with some modification and cleanup
  to support testing.
* Port over the eunit test suite from rebar2, but convert it to
  common_test. The testing is the same, but now the
  erlang term output is examined to determine if the test run is successful
  instead of scanning the console output for the expected strings.

support for provider_hooks in config

no more mode minimal

app_info:valid needs to return a boolean and not throw an exception

use processed app file for validation

fix typo in error tuple and throw error in install_deps

Clean up and backwards compatability

check locks before locking a dep to not have duplicates

add emulator args to escript for faster load time

don't lose erl_opts when compiling for tests

Error dep parsing

generalize some temporary directory handling and add it to `rebar_file_utils`

swap `file:list_dir` for `file:list_dir_all` due to nonexistence of latter prior to r16x

do provider returns the error of the first provider to fail and stops

no skip warning for seen dep that has locked version

(in `rebar_file_utils`)

change more instances of mistyped `erl_first_modules` to `erl_first_files`

`erl_first_files`

fixes #99