depgraph: tweak "required by" message format

The current output format for listing a chain of dependencies produces
one long flat line that can be hard to read.  For example, if you mask
dev-lang/ruby and then try to install dev-ruby/json, you'll see:
 The following mask changes are necessary to proceed:
  (see "package.unmask" in the portage(5) man page for more details)
 #required by dev-ruby/rdoc-3.12.1[ruby_targets_ruby19]
 # /etc/portage/package.mask/misc:
 =dev-lang/ruby-1.9.3_p385
 #required by dev-ruby/rubygems-1.8.24[ruby_targets_ruby18], required by virtual/rubygems-1, required by dev-ruby/racc-1.4.9[-test,ruby_targets_ruby18], required by dev-ruby/rdoc-3.12.1[ruby_targets_ruby18], required by dev-lang/ruby-1.9.3_p385[rdoc], required by dev-ruby/rake-0.9.2.2[ruby_targets_ruby19], required by dev-ruby/json-1.7.7[-test,-doc,ruby_targets_ruby19]
 # /etc/portage/package.mask/misc:
 =dev-lang/ruby-1.8.7_p371

Tracing your way through that list is not easy.  Instead, let's use
newlines and now we get:
 The following mask changes are necessary to proceed:
  (see "package.unmask" in the portage(5) man page for more details)
 # required by dev-ruby/rdoc-3.12.1[ruby_targets_ruby19]
 # /etc/portage/package.mask/misc:
 =dev-lang/ruby-1.9.3_p385
 # required by dev-ruby/rubygems-1.8.24[ruby_targets_ruby18]
 # required by virtual/rubygems-1
 # required by dev-ruby/racc-1.4.9[-test,ruby_targets_ruby18]
 # required by dev-ruby/rdoc-3.12.1[ruby_targets_ruby18]
 # required by dev-lang/ruby-1.9.3_p385[rdoc]
 # required by dev-ruby/rake-0.9.2.2[ruby_targets_ruby19]
 # required by dev-ruby/json-1.7.7[-test,-doc,ruby_targets_ruby19]
 # /etc/portage/package.mask/misc:
 =dev-lang/ruby-1.8.7_p371

Signed-off-by: Mike Frysinger <vapier@gentoo.org>