changelog: report through the log instead of printing to stdout

generate_entry's status messages become log lines, and the unmet
build-dependency diagnostics travel inside the UnmetBuildDependencies
error (its Display carries the full report) instead of being printed
to stderr by the library: the caller renders both like any other
outcome.

src/changelog.rs

@@ -20,7 +20,7 @@ pub fn generate_entry(
 
     // Parse existing changelog to get current (old) version
     let (package, old_version, series) = parse_changelog_header(&changelog_path)?;
-    println!("Found package: {}, version: {}", package, old_version);
+    log::info!("Found package: {}, version: {}", package, old_version);
 
     // Open git repo, and find commits since last version tag
     let repo_path = if let Some(path) = cwd {
@@ -56,7 +56,7 @@ pub fn generate_entry(
 
     prepend_to_file(&changelog_path, &new_entry)?;
 
-    println!("Added new changelog entry to {}", changelog_path.display());
+    log::info!("Added new changelog entry to {}", changelog_path.display());
 
     Ok(())
 }