Look for libraries' user header imports within . first

This avoids conflicts when all libraries dump their headers into the build folder.
Use HEADER_SEARCH_PATHS instead, to catch bracket imports too
Revert "Use HEADER_SEARCH_PATHS instead, to catch bracket imports too"

This reverts commit 0017753c97a928770fe9fdc82f6e9792058897b4.
Merge pull request #11 from jspahrsummers/project-headers-first

Look for user headers in the project folder first