patch 8.1.1979: code for handling file names is spread out

Problem:    Code for handling file names is spread out.
Solution:   Move code to new filepath.c file.  Graduate FEAT_MODIFY_FNAME.
diff --git a/src/feature.h b/src/feature.h
index 8afa987..c4ccd21 100644
--- a/src/feature.h
+++ b/src/feature.h
@@ -107,6 +107,7 @@
  * +multi_byte		Generic multi-byte character handling.
  * +cmdline_compl	completion of mappings/abbreviations in cmdline mode.
  * +insert_expand	CTRL-N/CTRL-P/CTRL-X in insert mode.
+ * +modify_fname	modifiers for file name.  E.g., "%:p:h".
  *
  * Obsolete:
  * +tag_old_static	Old style static tags: "file:tag  file  ..".
@@ -354,13 +355,6 @@
 #endif
 
 /*
- * +modify_fname	modifiers for file name.  E.g., "%:p:h".
- */
-#ifdef FEAT_NORMAL
-# define FEAT_MODIFY_FNAME
-#endif
-
-/*
  * +diff		Displaying diffs in a nice way.
  *			Requires +windows and +autocmd.
  */