prepare.cppm: un-anonymize the preamble helpers ahead of the file split

The preamble's internal helpers (warn_unknown_xpkg_keys through
with_index_cause, and the ToolPurpose enum among them) sat in an
anonymous namespace, giving them TU-local (internal) linkage. That was
fine while every phase using them lived in the same file; once the
phases move to their own implementation units, an internal-linkage
name is invisible outside the file that declares it (anonymous-
namespace scoping is per translation unit, modules or not). Dropped
the wrapper: these are now plain module-linkage declarations in
prepare.cppm, which -- like pending_flag_words_notes(), already
declared that way -- every implementation unit of the module can see
without needing to be exported. No behaviour change: nothing outside
the module could name these either way.

Golden fixture diff against the baseline is empty; mcpp test: 129/129.

Author: speak-agent

src/build/prepare.cppm

@@ -1526,7 +1526,6 @@ std::string git_cache_head(const std::filesystem::path& gitRoot) {
 //   extraTargets:   additional Target entries (e.g. synthetic test targets)
 //                   appended to the manifest before the modgraph runs.
 //   overrides:      --target / --static.
-namespace {
 // A dependency that "cannot be found" while an index is unreadable is almost
 // never missing — it is unreachable, and the two need different actions from
 // the user (publish it vs upgrade mcpp). The floor error is printed when the
@@ -2334,7 +2333,6 @@ std::string with_index_cause(std::string msg) {
         msg += "\n" + hint;
     return msg;
 }
-} // namespace
 
 // The toolchain a host tool's package chose for itself, read the way its own
 // build reads it (#710): the package's manifest with the root-position keys of