fix: recompile the units whose std module was rebuilt in place

A .cpp unit's depfile listed its source and headers, never the BMIs it
imports: clang's driver adds -module-file-deps to --precompile only. A
std module rebuilt in its directory, which happens whenever its source
changes under one compiler and one set of flags (a libc++ update, or
mingw-w64's under the same prefix), left every object that imports it
as it was, and the next build only relinked them. Implementation units
are now compiled with -fmodule-file-deps, so the std PCM and the
project's own PCMs are prerequisites of their objects like headers.

Depfiles written before this lack the edge, so a std module that moves
directories still needs VariantId to move with it (StdPcmKey).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

implementations/Crafter.Build-Implementation.cpp

@@ -51,8 +51,13 @@ namespace Crafter {
             return;
         }
 
-        // -MD leaves <name>_impl.o.d beside the object for Check to read.
+        // -MD leaves <name>_impl.o.d beside the object for Check to read, and
-        std::string result = RunBuildCommand(std::format("{0} {1}.cpp -c -MD -MF {2}_impl.o.d -o {2}_impl.o", clang, path.string(), (buildDir/path.filename()).string()));
+        // -fmodule-file-deps puts the BMIs the unit imported in it with the
+        // headers. The driver adds that for --precompile only, so a std
+        // module rebuilt in place, for a mingw-w64 or libc++ update under
+        // the same compiler, left the objects importing it as they were, and
+        // the next build only relinked them.
+        std::string result = RunBuildCommand(std::format("{0} {1}.cpp -c -MD -MF {2}_impl.o.d -fmodule-file-deps -o {2}_impl.o", clang, path.string(), (buildDir/path.filename()).string()));
 
         bool expected = false;
         if(!result.empty() && buildCancelled.compare_exchange_strong(expected, true)) {