Support DEPENDS_ARGS/FLAVORS - Building multiple packages from one origin

[bdrewery]

This fixes #259 and #313, and obsoletes #314.

Remaining items:

1. The next thing will be to add FLAVORS support for https://reviews.freebsd.org/D10327. This should mostly be trivial.
2. Support ORIGIN@FLAVOR syntax for bulk -f list and bulk port, etc via listed_ports
   Ensure that ORIGIN ORIGIN@FLAVOR1 ORIGIN@FLAVOR2 works together.
3. Support RUN_DEPENDS+= foo>0:devel/foo:FLAVOR=flave
4. Make is_bad_flavor_slave_port requeue the main port with the proper DEPENDS_ARGS=PYTHON_VERSION for the originally request port. This will relieve the pressure to commit FLAVORS support immediately to prevent breaking all of the python ports listed.
5. The chosen FLAVOR or DEPENDS_ARGS is not always clear in the web interface but should be computable from the log. Not sure I will modify this before merge. (Will handle in JSON/HTML tracks by origin but should be pkgname #415)
6. All cache_get_origin need auditted to ensure they shouldn't be using originspec, like bulk showing ports to build.
7. The reinjection of items into the queue for DEPENDS_ARGS and FLAVORS needs to be unified for simplicity. moved to FLAVORS and DEPENDS_ARGS support is insane #492
8. Similar to (7), FLAVORS and DEPENDS_ARGS should not be supported set together.
9. Support origin@- to only build the first default flavor.
10. Need a test suite desperately. There's so many test cases here that keep failing when I change things. Need automated bulk tests #488
11. Fix the CHECK_CHANGED_DEPS support for ORIGIN@FLAVOR dependency lines. It currently will bomb since the packages only record ORIGIN. Need to convert to PKGBASE or something. This code belongs in ports badly. CHECKED_CHANGED_DEPS support for originspecs #495

The python slave port issue is quite complex.
If a port like net/tiny-network-utilities requires net/py-netifaces as python3 it currently does not work due to the DEPENDS_ARGS bug. The workaround today in ports is to change the dependency to a new slave port such as net/py3-netifaces. In this specific example net/tiny-network-utilities does not depend on a py3 slaveport. Once Poudriere is merged with DEPENDS_ARGS support it, with bulk -a will then queue the virtual net/py-netifaces for python2 (default) and then also python3 from the net/tiny-network-utilities DEPENDS_ARGS dependency. It will also queue a build for net/py3-netifaces. The slave port and the dependency-generated package will both be 'py3-netifaces' for PKGNAME and thus hit the duplicated PKGNAME assertion and fail to build.
To work around this I force mapped origins that had a slave port added but would build just fine now from the main port from a DEPENDS_ARGS-generated package. The problem that comes from all of this is that if a py3 slave port was added not for a dependency solution but purely to provide a py3 package for a user. In that case without FLAVORS a py3- package will never generate from the default port. So merging this into the package builds will drop a lot of py3- packages. That is fixed by merging FLAVORS support to the python ports which then will ensure all main python ports provide a py2 and py3 package from the origin whether they are a dependency or not.

Performance of bulk -a startup is hurt by 7% but it's relatively fast still.

x before
+ after
+------------------------------------------------------------+
|                                                           +|
|   x                                                       +|
| x x      x                                                +|
||__MA___|                                                   |
|                                                           A|
+------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x   4        275.69        279.24        276.52      276.9925     1.5485773
+   3           297           297           297           297             0
Difference at 95.0% confidence
        20.0075 +/- 2.35542
        7.22312% +/- 0.911779%
        (Student's t, pooled s = 1.19952)