Add sample files for testcases

tests/cases/projects/declarationDir/a.ts

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+import {B} from './subfolder/b';
+export class A {
+    b: B;
+}

tests/cases/projects/declarationDir/subfolder/b.ts

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+export class B {
+    
+}

tests/cases/projects/declarationDir/subfolder/c.ts

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+import {A} from '../a';
+
+export class C {
+    a: A;
+}