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

In a test on #4810 (comment)
Before we got to 1700000 ticks in ~(82 to 97) sec
After we got to 1700000 ticks in ~(63 to 67) sec

String interning

This builds on the work from #5118. This interns the strings in the part of resolver that gets cloned a lot.

In a test on #4810 (comment)
Before we got to 1700000 ticks in ~(63 to 67) sec from #5118
After we got to 1700000 ticks in ~(42 to 45) sec

The interning code itself would be much better with a `leak` function that converts a `String` to a `&'static str`. Something like:
```rust
pub fn leek(s: String) -> &'static str {
    let ptr = s.as_ptr();
    let len = s.len();
    mem::forget(s);
    unsafe {
        let slice = slice::from_raw_parts(ptr, len);
        str::from_utf8(slice).unwrap()
    }
}
```
but "there is no unsafe in Cargo", and I am not the best at unsafe. So I just `to_string` and lived with the extra copy. Is there a better way to hand out references?

I assumed that `InternedString::new` world start appearing in profile result, and that we would want `PackageId`, and `Summary`, Et Al. to store the `InternedString`. That is why I put the interner in a shared folder. So far it is just used in the resolver. It may make sense for a lot more of the Strings to be interned, but with the extra copy... I have not explored it yet.

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

This is the same idea as #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 #4810 (comment) not to solve instantly.
Before 17000000 ticks, 90s, 188.889 ticks/ms
After 17000000 ticks, 73s, 232.877 ticks/ms