Allow sources to access their element's variables

Summary

Sometimes, it can be useful to define a 'version' or another variable that both source(s) and the element need, in order to not repeat oneself.

An example of that could be if trying to build openjdk:

kind: make

variables: version: 232 build: 05

sources:

• kind: tar url: openjdk:jdk8u/jdk8u/archive/jdk8u232-b05.tar.bz2 ref: ed87467e24c623396eae0bb2057bc2a8b8fd0ed30843e9413e578c7b3e5c689f
• kind: tar url: openjdk:jdk8u/jdk8u/langtools/archive/jdk8u232-b05.tar.bz2 ref: 812c70931157f5ba41e178b6e394a8552f15f53d0171d2f1b66537c9a76d5cc9 directory: langtools
• kind: tar url: openjdk:jdk8u/jdk8u/hotspot/archive/jdk8u232-b05.tar.bz2 directory: hotspot ref: 820da1b5dcfece8f249395b45983984c6c82886d78cbd1183faa98ad5497e8b8

config: configure-commands:

• | bash %{build-root}/configure
  --with-freetype-include=/usr/include/freetype2
  --with-freetype-lib=/usr/lib/x86_64-linux-gnu
  --with-update-version=%{version}
  --with-build-number=%{build}

Notice how the '232' and the '05' are duplicated in each source, in addition to being needed in the element.