`common_test` and `eunit` providers

option handling

in presence of keys defined multiple times

compiles common test files in a temporary dir (or a user specified
dir) and runs tests without leaving artifacts in the source tree

precompile tests using `rebar.config` and disable autocompile so
project specific options are used

fixes #62

compiles apps and eunit test files in a temporary dir (or a user
specified dir) and runs tests without leaving artifacts in the
source tree

automatically defines the 'TEST' macro if not already defined and
adds `test` to `src_dirs` to allow for eunit test files outside `src`

compiled first

- Includes tests for templates and their application

Now supports arbitrary commands

Using a namespace such as 'rebar3 lfe new' will look for the 'new'
command in 'lfe' namespace without the need for a 'do' provider to be
registered in that namespace.

Manually checked that 'rebar3 as <profile> <namespace> command' works,
as well with 'default' and 'do'.

Test suite still needed.

And more general namespace support

No longer needed.

- Adding tests for package deps
- Adding conflict/override warnings for package deps
- Adding cycle detection for packages
- Adding cycle detection for mixed packages+source
- Fixing internal dependency format of package resources when converted
  to rebar_app_info
- normalizing level-order prioritization to be based on lexicographical
  sort of app names rather than traversal order (which is undefined for
  package deps)
- Fixing tests for source deps for deep cycle detection
- Fixing bugs with source deps
- Relaxed version format checks for test utils

A lot of fixes are combined in there because I didn't want to commit
non-passing code, and many bugs were found when adding the second batch
of tests as part of the original effort.

The case in mind here is due to conflicts, and tests have been added for
this.

- Adding tests
- fixing use of set fetching to find repeated deps and prevent infinite
  loops

On a circular loop rebar3 now fails with `{error, no_sort}`, which is
uncaught and should be handled to consider the issue fully fixed.