Expose package orderers in rez config

[rlessardrodeofx]

This PR add a package_orderers entry in the rez config. Fixes #329.

This allow package orders to be configured.
We deployed this at RodeoFX 6 month ago and had no issues.
Our use case was to set a per_facility orderer so that rez "prefer" python 2 over python 3 by default.

Here's what our configuration look like:

package_orderers:
- orderers:
  - first_version: 2.7.16-r999
    packages:
    - python
    type: version_split
  type: per_family

Changes

• Added "package_orderers" entry in rez config
• Add a convenience class PackageOrderList (similar to PackageFilterList)
• Fix a serialization issue with VersionSplitPackageOrder (second point in Orderer fixes #413)
• Create a dedicate test_package_orders.py for package orderers tests (100% branch coverage on package_orderers.py. Note that test_9 in test_packages.py was moved into individual per-family tests.
• Minor: Make the TimestampPackageOrder rank default to 0 if unset

[JeanChristopheMorinPerso]

I (I work with @renaudll) confirm that we have been using this for 6 months and it's been working fine.