package definition question: referring to source files of another package?

[Andy Tai <atai@atai.org>]

Hi, this is a question for writing package definition.

In some scenarios package A may refer to source files in package B.
The question is,
1. How to reference source directory of package B from within
definition of package A?
2. can we even assume sources of Package B is available?   Normally if
package B is an input (dependency) for package A, guix would ensure
its installation directory is available, but not necessarily the
source.  So is there something that has to be done to ensure Package B
sources are somewhere?

Thanks for info on these.