refactor: Pun index logic to crates-index

This was inspired by running into errors where `cargo add inquire` was
not adding the latest version.  We've seen similar problems during the
development of `crates-index`, so hoping what they've done fixes this.

Even if it doesn't solve this problem, hopefully by nature of using a
shared crate means it will get more use and more eyes to make it more
reliable in the long term.

Handling retrying the index is annoying.  I opened
frewsxcv/rust-crates-index#62

Note: in the next version of crates-index
- Opening an index will implicitly create it, so we won't be able to
  differentiate in our message whether we are initializing or updating.
  See frewsxcv/rust-crates-index#61
- We will be able to open an Index by URL, which will allow us to remove
  more logic

Cargo.toml

@@ -46,6 +46,7 @@ required-features = ["upgrade"]
 [dependencies]
 atty = { version = "0.2.14", optional = true }
 cargo_metadata = "0.14.0"
+crates-index = "0.17.0"
 dirs-next = "2.0.0"
 env_proxy = "0.4.1"
 error-chain = "0.12.4"

src/errors.rs

@@ -5,6 +5,8 @@ error_chain! {
         Io(::std::io::Error) #[doc = "An error from the std::io module"];
         Git(::git2::Error)#[doc = "An error from the git2 crate"];
         CargoMetadata(::failure::Compat<::cargo_metadata::Error>)#[doc = "An error from the cargo_metadata crate"];
+        Semver(::semver::Error)#[doc = "An error from the semver crate"];
+        CratesIndex(::crates_index::Error)#[doc = "An error from the crates-index crate"];
     }
 
     errors {