Combine outputs from several packages to a shared location

["Simen Endsjø" <contact@simendsjo.me>]

Hi, I'm trying to package some software where I combine various packages into a
single location. I'll try to explain...

I have a package P. This package contains a binary, B, some libraries S, and
some libraries R. These are usually packages together, in which case things just
work, but I need to support adding various versions of some of these packages.

E.g. I add P@2:out to get everything for that version, but additionally P@1:S
and P@1:R.  B can only be added once as it is at the same location, and we
usually use the latest version, but S and R have versioned subdirectories. This
means S@1 is located at /s/1 and R@2 is located at /r/2.

Given the shell with P@2:out, P@1:S, P@1:R, I should get a structure like

/B
/s/1
/s/2
/r/1
/r/2

This is the structure you get when installing the packages on other distros and
operating systems.

But how can I create such a "virtual package"? I also need to register this
virtual package path in an environment variable so B is able to locate
all S and R versions, P_ROOT.

Right now only :out works as I cannot get the locations of other P:S and P:R
known to B. The single environment variable can only point at a single path, and
that path must include the s and r directories with all installed versions.

How can I solve this problem?
Are there other packages which does something similar I might look at?

Regards Simen