Auto merge of #5302 - Eh2406:MoreRc, r=alexcrichton

use more Rc in the part of resolver that gets cloned a lot 2

This is the same idea as https://github.com/rust-lang/cargo/pull/5118, I was looking at a profile and noted that ~5% of our time was sent dropping `HashMap<PackageId, HashSet<InternedString>>`. A quick rg and I found the culprit, we are cloning the set of features for every new `Context`. With some Rc we are now just cloning for each time we insert.

To benchmark I commented out line https://github.com/rust-lang/cargo/blob/b9aa315158fe4d8d63672a49200401922ef7385d/src/cargo/core/resolver/mod.rs#L453
the smallest change to get https://github.com/rust-lang/cargo/issues/4810#issuecomment-357553286 not to solve instantly.
Before 17000000 ticks, 90s, 188.889 ticks/ms
After 17000000 ticks, 73s, 232.877 ticks/ms