Provide a workaround to ignore bad indices

[ymmt2005]

Some repositories are badly configured to return wrong contents for uncompressed indices.

For instance, pkg.jenkins.io returned wrong Packages as pointed by @takumin:
#25 (comment)

Zabbix also returned wrong Sources for which go-apt-mirror implements a workaround already:

aptutil/mirror/mirror.go

Lines 243 to 257 in 66c646f

	// WORKAROUND: some (zabbix) repositories returns wrong contents
	// for non-existent files such as Sources (looks like the body of
	// Sources.gz is returned).
	if !m.mc.Source {
	tmpMap := make(map[string][]*apt.FileInfo)
	for p, fil := range indexMap {
	base := path.Base(p)
	base = base[0 : len(base)-len(path.Ext(base))]
	if base == "Sources" {
	continue
	}
	tmpMap[p] = fil
	}
	indexMap = tmpMap
	}

Although these indices are bad, apt or apt-get will work nicely as they look compressed indices
such as Packages.bz2 or Sources.gz first.

To help users of go-apt-mirror, we should provide a general workaround for this problem.