tags: replace dots by slashes in file path to hierarchical modules

The itask-tonic compiler generates tags files. This also includes stuff from the standard library, in particular functions in hierarchical modules. A tag includes the name of the file where the identifier is defined. When the identifier is in a hierarchical module, the file name contains dots in places where a directory delimiter should be.

For example, the function forM in Control.Monad. The dot in the module name Control.Monad needs to be replaced by a directory separator /.

Currently, the file name for the tag generated for forM looks like this:

/home/mkl/clean/clean-svn-2016-04-15/lib/iTasks-SDK/Dependencies/clean-platform/src/libraries/OS-Independent/Control.Monad.icl

while it should look like this:

/home/mkl/clean/clean-svn-2016-04-15/lib/iTasks-SDK/Dependencies/clean-platform/src/libraries/OS-Independent/Control/Monad.icl