refactor: simplify the deserializer of PyPiRequirement

[orhun]

closes #621

[orhun]

P.S. I also tried:

            .map(|map| {
                PyPiRequirement::deserialize(de::value::MapAccessDeserializer::new(map))
                    .map_err(Error::custom)
            })

which resulted in stack overflow 💀

thread 'project::manifest::python::tests::test_deserialize_pypi_requirement_from_map' has overflowed its stack
fatal runtime error: stack overflow

So I kept the logic.

[ruben-arts]

LGTM!