Sourcing: support basic variable expansions

[skovhus]

#244 introduced sourcing aware symbol resolution where source commands (e.g. . myfile.sh or source ./file.bash) is used for figure out which symbols are available in a given context.

The current implementation is fairly crude and can be extended a bit to resolve basic variable expansions and sourcing statements in functions and loops.

[skovhus]

With #673 we now just miss basic variable expansion support.