trimFileName no longer works in a monorepo

[ecraig12345]

#392 changed the behavior of trimming filenames so that it no longer works properly on types coming from node_modules in a monorepo, where commands may be run from a particular package but dependencies are installed at the repo root.

For example, I might run commands from an individual package /Users/me/my-repo/packages/foo, but the React typings are installed at the root, /Users/me/my-repo/node_modules/@types/react/index.d.ts.

Previously, the parent/declaration filename for props derived from React types would come back as my-repo/node_modules/@types/react/index.d.ts. Then we used a propFilter to exclude props coming from that file.

After #392, the filename in this case is just index.d.ts, which breaks the filtering.