refactor: Remove VersionExt

This came out of my work on rust-lang#12801.
I was looking at what might be appropriate to put in a `cargo-util-semver` crate and realized we have the `VersionExt` trait that exists but doesn't do much, so I dropped it.

src/cargo/core/resolver/errors.rs

@@ -4,7 +4,7 @@ use std::task::Poll;
 use crate::core::{Dependency, PackageId, Registry, Summary};
 use crate::sources::source::QueryKind;
 use crate::util::edit_distance::edit_distance;
-use crate::util::{Config, OptVersionReq, VersionExt};
+use crate::util::{Config, OptVersionReq};
 use anyhow::Error;
 
 use super::context::Context;
@@ -275,7 +275,7 @@ pub(super) fn activation_error(
         msg.push_str(&describe_path_in_context(cx, &parent.package_id()));
 
         // If we have a pre-release candidate, then that may be what our user is looking for
-        if let Some(pre) = candidates.iter().find(|c| c.version().is_prerelease()) {
+        if let Some(pre) = candidates.iter().find(|c| !c.version().pre.is_empty()) {
             msg.push_str("\nif you are looking for the prerelease package it needs to be specified explicitly");
             msg.push_str(&format!(
                 "\n    {} = {{ version = \"{}\" }}",

src/cargo/util/mod.rs

@@ -22,7 +22,7 @@ pub use self::progress::{Progress, ProgressStyle};
 pub use self::queue::Queue;
 pub use self::restricted_names::validate_package_name;
 pub use self::rustc::Rustc;
-pub use self::semver_ext::{OptVersionReq, PartialVersion, RustVersion, VersionExt, VersionReqExt};
+pub use self::semver_ext::{OptVersionReq, PartialVersion, RustVersion, VersionReqExt};
 pub use self::to_semver::ToSemver;
 pub use self::vcs::{existing_vcs_repo, FossilRepo, GitRepo, HgRepo, PijulRepo};
 pub use self::workspace::{

src/cargo/util/semver_ext.rs

@@ -12,20 +12,10 @@ pub enum OptVersionReq {
     UpdatePrecise(Version, VersionReq),
 }
 
-pub trait VersionExt {
-    fn is_prerelease(&self) -> bool;
-}
-
 pub trait VersionReqExt {
     fn exact(version: &Version) -> Self;
 }
 
-impl VersionExt for Version {
-    fn is_prerelease(&self) -> bool {
-        !self.pre.is_empty()
-    }
-}
-
 impl VersionReqExt for VersionReq {
     fn exact(version: &Version) -> Self {
         VersionReq {