Use main module pod file by default if available

If the main module has a companion pod file with the same basename, that
pod file will be used in preference to the main module file itself as
the source of the pod for the readme. This only affects the default and
has no effect if the source_filename option is specified explicitly in
the config.
v0.142470

    - Use main module pod file by default if available. If your main
      module has its POD in a separate file with the same basename and
      ".pod" extension, then that file will be used automatically as
      the default.