Conditional features for conditional dependencies

[elinorbgr]

In short, being able to reflect this kind of dependencies of features:

• if feature foo is enabled, depend on crate krate
  • if additionally feature bar is enabled, depend on krate/bar
• if feature foo is not enabled, don't depend on krate, even if bar is enabled

I don't see any way to reflect this currently:

• if bar depends on krate/bar, enabling it will always pull krate, even if foo is not enabled
• if it doesn't, enabling foo and bar does not pull krate/bar, which in my case would cause a compilation error

[Nemo157]

Duplicate of #3494

[ehuss]

Yea, looks the same, thanks!