Unify Fortran sources and preprocessor include directories

[gnikit]

I think that using a separate directory for the preprocessor includes, which by design, does not include any of the source directories other than CWD is counterintuitive and should not be the norm.

I think that passing source files (including preprocessor files) should be a single option. More extensions for source files can be specified with the option incl_suffixes. For the cases were files that contain preprocessor commands, that we don't want to parse, with extensions matching those of incl_suffixes and are randomly scattered in the source tree i.e. we cannot exclude them with any of the exclude patterns should not be supported.

fortls, like almost all Language Servers, assumes some logical ordering of one's project, where files that are not needed have been somehow sanitised. The main comparison that I am drawing is the LS of C/C++.