Fix feature conflicts v2

[marioortizmanero]

Description

This supersedes #222, which is now kinda outdated.

In this attempt I'm trying to follow what the https://github.com/bodil/im-rs crate did. We can just have two crates; the main one for async, and a separate one for sync. We can keep using maybe_async in theory. This is unfortunately a bit confusing but it's the only way to do it. I'm working on a blog post with research about the whole async & sync stuff and I'll post it before this PR is merged to give more context and a full explanation of how it works.

Motivation and Context

This closes #221

Dependencies

None

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

An empty project with this Cargo.toml will fail:

[package]
name = "conflict"
version = "0.1.0"
edition = "2018"

[dependencies]
# Method number one
rspotify = { git = "https://github.com/ramsayleung/rspotify.git", branch = "master", features = ["client-reqwest", "client-ureq"]}

# Method number two
# rspotify-sync = { git = "https://github.com/ramsayleung/rspotify.git", branch = "fix-conflicts", features = ["client-reqwest", "client-ureq"] }
# rspotify-async = { git = "https://github.com/ramsayleung/rspotify.git", branch = "fix-conflicts", features = ["client-reqwest", "client-ureq"] }

But with this it will work:

[package]
name = "conflict"
version = "0.1.0"
edition = "2018"

[dependencies]
# Method number one
# rspotify = { git = "https://github.com/ramsayleung/rspotify.git", branch = "master", features = ["client-reqwest", "client-ureq"]}

# Method number two
rspotify-sync = { git = "https://github.com/ramsayleung/rspotify.git", branch = "fix-conflicts-2", features = ["client-ureq"] }
rspotify-async = { git = "https://github.com/ramsayleung/rspotify.git", branch = "fix-conflicts-2", features = ["client-reqwest"] }

[marioortizmanero]

Nah, I haven't managed to get this working either... Even though there are two separate crates, a project with both rspotify-async and rspotify-sync will fail to compile. Even with different rev values (meaning that the crates shouldn't really be merged, because they are from different sources).