Adds the contents of a file to the table.
Looks up the given filename from the internal file buffer table, and returns the lines within the file. If the file does not already exist within the table, it will be read from the filesystem. If it has been read before,
Look for the source file if it's different from filename. Look for .di, .d, directory, and along global.path. Does not open the file.
Looks up the given filename from the internal file buffer table. If the file does not already exist within the table, it will be read from the filesystem. If it has been read before,