Rename path_to_url to file_path_to_url.

Just to make clear that it isn't appropriate for directories, for which
it would need to add a trailing slash. I don't want to add such a check.

src/actions.cpp

@@ -1892,7 +1892,7 @@ namespace quickbook
                     path = path_difference(state.xinclude_base, full_path);
                 }
 
-                return xinclude_path(full_path, detail::path_to_url(path));
+                return xinclude_path(full_path, detail::file_path_to_url(path));
             }
 
             default:

src/native_text.cpp

@@ -111,7 +111,7 @@ namespace detail {
     //
     // Some info on file URLs at:
     // https://en.wikipedia.org/wiki/File_URI_scheme
-    std::string path_to_url(fs::path const& x)
+    std::string file_path_to_url(fs::path const& x)
     {
         // TODO: Maybe should throw an exception if this doesn't understand the path.
         // TODO: Might need a special cygwin implementation.

src/native_text.hpp

@@ -125,7 +125,7 @@ namespace quickbook
         std::string path_to_generic(fs::path const&);
         fs::path generic_to_path(boost::string_ref);
 
-        std::string path_to_url(fs::path const&);
+        std::string file_path_to_url(fs::path const&);
 
         void initialise_output();