feat(cli): add verify-conditions subcommand

The verify-conditions subcommand implements the verifyConditions step
for sementic release for a Cargo-based rust workspace. It checks the
following conditions:

1. CARGO_REGISTRY_TOKEN is set and non-empty.
2. It can construct the dependencies graph for the workspace.
3. The dependencies graph in the workspace does not contain and links
that would inhibit the publication of the packages in the workspace.

Cargo.toml

@@ -14,3 +14,5 @@ structopt = "0.3.15"
 anyhow = "1.0.31"
 thiserror = "1.0.20"
 guppy = "0.5.0"
+itertools = "0.9.0"
+toml_edit = "0.2.0"

src/error.rs

@@ -9,9 +9,12 @@
 use std::io;
 use std::path::PathBuf;
 
+use guppy::graph::PackageLink;
 use guppy::errors::Error as GuppyError;
 use thiserror::Error;
 
+use super::DependencyType;
+
 /// The error type for operations `sementic-release-rust` operations.
 #[derive(Debug, Error)]
 pub enum Error {
@@ -22,6 +25,28 @@ pub enum Error {
     /// Error while writing to the output.
     #[error("Unable to write to the output")]
     OutputError(#[source] io::Error),
+
+    /// Error while verifying the conditions for a release.
+    #[error("Conditions for a release are not satisfied: {reason}")]
+    VerifyError{
+        /// The reason the conditions are not satisfied.
+        reason: String,
+    },
+
+    /// Error while verifying that dependencies allow publication.
+    ///
+    /// This is a specific part of verifying the conditions for a release.
+    #[error("{typ} of {from} on {to} prevents publication of {from}")]
+    BadDependancy{
+        /// The name of the package whose dependency prevents publication.
+        from: String,
+
+        /// The depended on package that prevents publication.
+        to: String,
+
+        /// The type of dependency that prevents publication.
+        typ: DependencyType,
+    }
 }
 
 /// A specialized `Result` type for `semantic-release-rust` operations.
@@ -47,4 +72,19 @@ impl Error {
     pub(crate) fn output_error(inner: io::Error) -> Error {
         Error::OutputError(inner)
     }
+
+    pub(crate) fn verify_error(reason: impl Into<String>) -> Error {
+        Error::VerifyError {
+            reason: reason.into(),
+        }
+    }
+
+    pub(crate) fn bad_dependency(link: &PackageLink, typ: DependencyType) -> Error {
+        Error::BadDependancy {
+            from: link.from().name().to_string(),
+            to: link.to().name().to_string(),
+            typ,
+        }
+    }
 }
+