Fix: make sure that source assemblies are attached to build

Fix: make sure that source assemblies are attached to build

Previously I renamed the source assemblies from the uniform name
'sources' to something more individual like 'aspectjtools-sources', not
realising that the magic name 'sources' in combination with the default
configuration value 'appendAssemblyId=true' results in an artifact
classifier equal to the assembly ID, i.e. 'sources', which is exactly
what we need here, but not quite obvious. Therefore, I documented it
with comments in both the assemblies and the POMs.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>