simplify autocompilation some more

simplify autocompilation some more

* libguile/load.c (scm_init_load_path): Set the fallback path even if
  GUILE_SYSTEM_COMPILED_PATH is set. Now that we store full paths in the
  autocompiled files, and the path contains the effective Guile version,
  there's no danger of accidental collisions.
  (do_try_autocompile, autocompile_catch_handler, scm_try_autocompile):
  Simplify again -- since there's only one place we put autocompiled
  files, and compile-file finds it itself, there's no need to pass along
  the compiled file path.
  (scm_primitive_load_path): Don't call out to compiled-file-name to get
  the fallback path, as we might not be autocompiling, and besides that
  we need to check if the file exists at all.

* module/system/base/compile.scm (compiled-file-name): Simplify again.
  The auto-compiled path is just fallback path + full source path + .go.