Cleanup: Move blend file path utilities out of blenloader module

These are not really about reading or writing .blend files, they are
general utilities for file-names/paths. Having to link to the
blendloader library just for these utilities is annoying.
Moved them to `BKE_blendfile.h` now, in agreement with Bastien.

@@ -22,0 +33,4 @@
+ *
+ * \param path: the full path to explode.
+ * \param r_dir: the string that'll contain path up to blend file itself ('library' path).
+ * WARNING! Must be #FILE_MAX_LIBEXTRA long (it also stores group and name strings)!

@@ -22,0 +35,4 @@
+ * \param r_dir: the string that'll contain path up to blend file itself ('library' path).
+ * WARNING! Must be #FILE_MAX_LIBEXTRA long (it also stores group and name strings)!
+ * \param r_group: the string that'll contain 'group' part of the path, if any. May be NULL.
+ * \param r_name: the string that'll contain data's name part of the path, if any. May be NULL.

@@ -65,0 +92,4 @@
+    return false;
+  }
+
+  strcpy(r_dir, path);