Provide a way to cleanly stage chiselled packages

[lengau]

What needs to get done

Include chisel and other packages needed for staging packages.

Why it needs to get done

Occasionally a charm (e.g. postgres-operator) needs to include executables or libraries to be installed on the system. In cases like this, they should be able to be installed using the craft-parts stage-packages mechanism

[lengau]

Also needs to set up craft-parts for ignored packages.