Add support for custom templates in subdirectories

- Includes tests for templates and their application

add erlydtl namespace

Provider namespaces

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.

add two cool dudes to the THANKS

Ensure dir for plt exists when copying.

don't include all */ebin dirs in code path

rewrite profiles

Test packages

- 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.

Print warning when deps are being skipped.

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

use PRV_ERROR to wrap a provider error in {error, {?MODULE, ...}}

version in source dep element is no longer used, support leaving it out

Fixes for profiles breaking templates and installing of non-default deps

Partial fix to circular deps (#40)

- 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.

if already seen dep before don't try handling it again

Update error message and fix order of dir creation for deps