fix(register): extension-aware module resolution and format handling

packages/register/esm.mts

@@ -31,11 +31,16 @@ const host: ts.ModuleResolutionHost = {
   fileExists: ts.sys.fileExists,
   readFile: ts.sys.readFile,
 }
+const EXTENSIONS: string[] = [ts.Extension.Ts, ts.Extension.Tsx, ts.Extension.Mts]
 
 export const resolve: ResolveFn = async (specifier, context, nextResolve) => {
   // entrypoint
   if (!context.parentURL) {
-    return { format: 'ts', url: specifier, shortCircuit: true }
+    return {
+      format: EXTENSIONS.some((ext) => specifier.endsWith(ext)) ? 'ts' : undefined,
+      url: specifier,
+      shortCircuit: true,
+    }
   }
 
   // import/require from external library
@@ -51,16 +56,23 @@ export const resolve: ResolveFn = async (specifier, context, nextResolve) => {
     moduleResolutionCache,
   )
 
-  // import from local project to local project
-  if (resolvedModule && !resolvedModule.isExternalLibraryImport) {
+  // import from local project to local project TS file
+  if (
+    resolvedModule &&
+    !resolvedModule.resolvedFileName.includes('/node_modules/') &&
+    EXTENSIONS.includes(resolvedModule.extension)
+  ) {
     return {
       format: 'ts',
       url: pathToFileURL(resolvedModule.resolvedFileName).href,
       shortCircuit: true,
     }
   }
 
-  // import from local project to external library (or unknown something)
+  // import from local project to either:
+  // - something TS couldn't resolve
+  // - external library
+  // - local project non-TS file
   return nextResolve(specifier)
 }