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/build/mod.rs

@@ -389,7 +389,9 @@ pub fn run_source_build(
         };
         let report = crate::debian::deps::check_build_depends(&ctrl, &check_opts)?;
         if !report.is_ok() {
-            eprintln!("{}", report.message());
+            // The diagnostics travel inside the error (its Display carries
+            // report.message()); main renders it and maps the type to exit
+            // status 3, like dpkg-buildpackage.
             return Err(Box::new(crate::debian::deps::UnmetBuildDependencies(
                 report,
             )));

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(())
 }

src/debian/deps.rs

@@ -890,7 +890,11 @@ pub struct UnmetBuildDependencies(pub UnmetReport);
 
 impl std::fmt::Display for UnmetBuildDependencies {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        write!(f, "build dependencies/conflicts unsatisfied; aborting")
+        write!(
+            f,
+            "build dependencies/conflicts unsatisfied; aborting\n{}",
+            self.0.message()
+        )
     }
 }