rez-pip does not create variants with a consistent order, potentially causing duplicates.

[jopollack]

Unsure of what specific code is going wrong under the hood, but rez-pip is clearly not ordering the list of packages in its variants prior to creating the package, which can create duplicates with slightly different ordering. In this case, I installed google_api_core twice, and was confused when the second wasn't ignored for already existing. Then I noticed the variants looked like this:

variants = [
    ['python-2.7', 'protobuf-3.12.0+<3.18.0', 'futures-3.2.0+'],
    ['python-2.7', 'futures-3.2.0+', 'protobuf-3.12.0+<3.18.0']
]

Environment

• OS Linux CentOS7
• Rez version 2.106.0
• Rez python version 3.7.7

To Reproduce

1. rez-pip google_api_core -i -p /temp_directory/rez --python-version 2.7
2. repeat the above again.

Expected behavior
The same install steps should be recognized as identical and skipped as already existing.

Actual behavior
The package gets installed again, with identical variants in a slightly different order, which I expect to cause unexpected and undesirable behavior when attempting to resolve.