Fully support key value pairs

[remkade]

We need to add full support for Key Value pairs. However, they should be lower precedence than attributes specified outside of the key value pairs.

Here's the list of ones we should allow:

• License
• Build
• Build number
• md5
• sha256

I'm definitely open to discussion on that list, but I don't want to encourage bad practice or confusion by supporting too many ways to specify things.

For example, these confusing ones:

python[name=java]  # would match java
python==3.10.0[name=* version=* md5='abcdef'] # installs a package of any name, any version, that matches the md5

Because of the opportunity for confusion here we should probably make the "normal" way of specifying things have higher precedence than the key value pair, or just not support keys that have a "normal" representation.

[razzlestorm]

License-family is still supported as a matching field in conda, should that key also be allowed? I personally don't think it has a lot of value in conda's current implementation because that allows regex, but it could be useful if we're not allowing any regex.