feat: Support PEP508 environment markers

[hameerabbasi]

Description

Fixes #1259

How Has This Been Tested?

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[tdejager]

Good work seems easier then expected! Can you test if satisfiability works as expected, i.e changing markers re-solves and such? Also test if adding a linux marker with windows as a platform for example creates an empty dependency set. Maybe there are more edge cases we could test and report on?

[tdejager]

Maybe we should also have some documentation updates, not sure, could you have a look?

[hameerabbasi]

Also test if adding a linux marker with windows as a platform for example creates an empty dependency set.

I changed the test to do this in 2a24e4b, but I have no idea how to fix it sadly. Soo many types and conversions my grepping abilities are failing me.

Edit: Added another test in 1e824d2.

[hameerabbasi]

Can you test if satisfiability works as expected, i.e changing markers re-solves and such?

Done in 06329e3

[hameerabbasi]

@tdejager @baszalmstra I think this is ready, review/merge appreciated.

[tdejager]

I looked through everything and all of it makes sense! Let me do a small user-test myself.