compiler: lock incremental dependency tracking behind --debug-incremental

This logic (currently) has a non-trivial cost (particularly in terms of
peak RSS) for tracking dependencies. Until incremental compilation is in
use in the wild, it doesn't make sense for users to pay that cost.

src/Module.zig

@@ -3139,6 +3139,8 @@ fn markDeclDependenciesPotentiallyOutdated(zcu: *Zcu, decl_index: Decl.Index) !v
 }
 
 pub fn findOutdatedToAnalyze(zcu: *Zcu) Allocator.Error!?InternPool.Depender {
+    if (!zcu.comp.debug_incremental) return null;
+
     if (zcu.outdated.count() == 0 and zcu.potentially_outdated.count() == 0) {
         log.debug("findOutdatedToAnalyze: no outdated depender", .{});
         return null;