Revert "Reland "refactor(core): cleanup feature flags for js source i… (#19611)

…nclusion" (#19519)"

This reverts commit 28a4f3d0f5383695b1d49ccdc8b0f799a715b2c2.

This change causes failures when used outside Deno repo:
```
============================================================
Deno has panicked. This is a bug in Deno. Please report this
at https://github.com/denoland/deno/issues/new.
If you can reliably reproduce this panic, include the
reproduction steps and re-run with the RUST_BACKTRACE=1 env
var set and include the backtrace in your report.

Platform: linux x86_64
Version: 1.34.3+b37b286
Args: ["/opt/hostedtoolcache/deno/0.0.0-b37b286f7fa68d5656f7c180f6127bdc38cf2cf5/x64/deno", "test", "--doc", "--unstable", "--allow-all", "--coverage=./cov"]

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Failed to read "/home/runner/work/deno/deno/core/00_primordials.js"

Caused by:
    No such file or directory (os error 2)', core/runtime/jsruntime.rs:699:8
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

1 files changed, 24 insertions, 0 deletions

diff --git a/core/modules/loaders.rs b/core/modules/loaders.rs
index fc0ab2deb..d4dbf1ec2 100644
--- a/core/modules/loaders.rs
+++ b/core/modules/loaders.rs
@@ -119,6 +119,7 @@ impl ExtModuleLoader {
       extensions
         .iter()
         .flat_map(|e| e.get_esm_sources())
+        .flatten()
         .map(|s| (s.specifier.to_string(), s.clone())),
     );
     ExtModuleLoader {
@@ -178,6 +179,29 @@ impl ModuleLoader for ExtModuleLoader {
   }
 }
 
+impl Drop for ExtModuleLoader {
+  fn drop(&mut self) {
+    let sources = self.sources.get_mut();
+    let used_specifiers = self.used_specifiers.get_mut();
+    let unused_modules: Vec<_> = sources
+      .iter()
+      .filter(|(k, _)| !used_specifiers.contains(k.as_str()))
+      .collect();
+
+    if !unused_modules.is_empty() {
+      let mut msg =
+        "Following modules were passed to ExtModuleLoader but never used:\n"
+          .to_string();
+      for m in unused_modules {
+        msg.push_str("  - ");
+        msg.push_str(m.0);
+        msg.push('\n');
+      }
+      panic!("{}", msg);
+    }
+  }
+}
+
 /// Basic file system module loader.
 ///
 /// Note that this loader will **block** event loop