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/quirks.rs

@@ -266,7 +266,7 @@ fn apply_rules(
     }
 
     for injected in &deps.inject {
-        let parsed = Deps::parse(injected, opts)?;
+        let parsed = Deps::parse(injected, opts).map_err(|e| e.to_string())?;
         clauses.extend(parsed.clauses().map(<[PkgRelation]>::to_vec));
     }