[ issue1241 ] Fixed issue #1241, still room for refactoring

Using Agda.Interaction.FindFile (findFile) to find the source file corresponding to the top-level module being typechecked.

Uses this as input FilePath for LaTeX generation. The output path is the latex-dir command-line option concatenated with the top-level module name "transformed" into a FilePath.

generateHTML seems to be doing the same we do here (transform a top-level module name into a path), and this might happen at other places too (compiler backends, etc.), so here's an opportunity for refactoring.