refactor(core): migrations do not properly handle multiple templates in source file (#29841)

Currently if there are multiple source files within a given
TypeScript source file, only the last template in the source
file is checked as we store templates in a `Map` with the
source file paths as keys.

This is problematic as multiple templates can live within the
same source file and we therefore accidentally overwrite
existing entries in the resolved templates map.

PR Close #29841

packages/core/schematics/test/template_var_assignment_migration_spec.ts

@@ -197,4 +197,26 @@ describe('template variable assignment migration', () => {
 
     expect(warnOutput.length).toBe(0);
   });
+
+  it('should be able to report multiple templates within the same source file', () => {
+    writeFile('/index.ts', `
+      import {Component} from '@angular/core';
+      
+      @Component({
+        template: '<ng-template let-one><a (sayHello)="one=true"></a></ng-template>',
+      })
+      export class MyComp {}
+      
+      @Component({
+        template: '<ng-template let-two><b (greet)="two=true"></b></ng-template>',
+      })
+      export class MyComp2 {}
+    `);
+
+    runMigration();
+
+    expect(warnOutput.length).toBe(2);
+    expect(warnOutput[0]).toMatch(/^⮑ {3}index.ts@5:56: Found assignment/);
+    expect(warnOutput[1]).toMatch(/^⮑ {3}index.ts@10:53: Found assignment/);
+  });
 });