Make persistFileWith use project-root relative paths

With code like this:

share [mkPersist sqlSettings, mkMigrate "migrateAll"]
    $(persistFileWith lowerCaseSettings "config/models")

If you run stack ghci from a directory other than the project root you get this error:

[ 2 of 13] Compiling Model            ( /home/enolan/mystuff/code/emp-pl-site/Model.hs, interpreted )

/home/enolan/mystuff/code/emp-pl-site/Model.hs:12:7:
    Exception when trying to run compile-time code:
      config/models: openFile: does not exist (No such file or directory)
    Code: persistFileWith lowerCaseSettings "config/models"
    In the splice: $(persistFileWith lowerCaseSettings "config/models")

I started a quick and dirty PR than just copy pastes makeRelativeToProject from file-embed in and uses it, but then I realized that's @snoyberg's package. Maybe file-embed should expose it? Or expose something like embedFileText :: FilePath -> Q Text?

Start by locating the implementation and call path for persistFileWith, then compare the mentioned makeRelativeToProject approach in file-embed. Reproduce the failure by running stack ghci outside the project root. Done means the persistFileWith example can resolve config/models from another working directory, with the API approach agreed by maintainers.