$DISTFILE_FETCHER configuration variable specifies what suite must be
used to download and verify integrity of distfiles.

By default, $DISTFILES/meta contains directories with various
metainformation about distfile: its size, hashes and URLs.
Initially Metalink4 XML files were used instead.
=> Metalink4
$DISTFILES/bin/meta4-to-metadir uses meta4ra utilities
to convert .meta4 file to "metadir".
$DISTFILES/bin/metadir-to-meta4 generates .meta4 back.

A [Index/Variables] JUSTDL
* $DISTFILE_FETCHER=justdl
  * Get list or possible download URLs. It is sorted with
    FETCHER_URLS_SORT function, which is by default:
      FETCHER_URLS_SORT() {
          $DISTFILES/lib/urls-sort "" rand
      }
  * For each URL, until we succeed, run $JUSTDL program to fetch it.
    If JUSTDL is not set, then BASS will check for following programs
    existence: fetch, wget, curl, meta4ra-dl.
  * Check downloaded result with $BASS_ROOT/build/bin/hashes-check
    utility against $metadir/hashes, if it exists. Otherwise check
    only the size.
* $DISTFILE_FETCHER=wget-meta4
  => GNU Wget
  Use wget compiled with --with-metalink option.

* $DISTFILE_FETCHER=aria2c
  => Aria2
  Also uses generated .meta4 files.
  Unfortunately it sometimes fails to deal with links on GitHub.com.