Ancestor directories should be lazily loaded

[dsherret]

For example, the following code:

const sourceFile = project.createSourceFile("V:/test/sub/dir/mySourceFile.ts");
const directory = sourceFile.getDirectory();
const testSubDir = directory.getParent();
const testDir = project.getSourceFile("V:/test/");

...currently returns undefined for both testSubDir and testDir. It would be good if these were lazily loaded upon request so as to not affect the behaviour of .getRootDirectories() until they're requested.