debian: type the errors of the format primitives

Give every parser of the shared format layer a proper error enum
(thiserror): VersionParseError, ArchError, deps::ParseError (wrapping
the version and arch failures as #[source] so the context chain
survives), ControlParseError, ChecksumParseError, FilesParseError and
TestsControlParseError, each carrying its structured pieces (the
offending string, the stanza number, the field name) instead of a
pre-formatted String.

Display texts are byte-identical to the old format! strings, so
user-facing output and the message-asserting tests are unchanged; the
few tests asserting on error text now render the error first. Flow
boundaries that still take String (build::env, quirks, test::expand)
render with to_string() at the edge — typing those flows is a later
step. ControlInfo's FromStr impl follows parse_content's error type.

src/test/expand.rs

@@ -114,7 +114,7 @@ fn split_top_level_commas(input: &str) -> Vec<String> {
 /// stripped") and keeping everything else verbatim.
 fn name_only_clauses(input: &str, host_arch: &str) -> Result<Vec<String>, String> {
     let opts = ParseOpts::build_deps(host_arch.to_string(), Vec::new());
-    let deps = Deps::parse(input, &opts)?;
+    let deps = Deps::parse(input, &opts).map_err(|e| e.to_string())?;
     let mut clauses = Vec::new();
     for clause in deps.clauses() {
         let kept: Vec<String> = clause