Fix compilation-find-file advice for dirs without direct files

[bbatsov]

Summary

• The compilation-find-file advice now also resolves filenames relative to the project root, adding the parent directory to the search path when found
• This fixes the edge case where a directory contains only subdirectories (no files directly), so it wasn't included in the file-derived directory list from projectile-current-project-dirs
• The fix is lightweight — a single file-exists-p check — avoiding the performance cost of a full recursive directory scan

Fixes #1923

Test plan

• Create a project with a nested directory structure where an intermediate directory contains only subdirectories
• Run a compilation that outputs file paths relative to that intermediate directory
• Click on the file path in the compilation buffer — it should navigate correctly instead of prompting