Add Make variable for target package source dir

[mering]

Description of the feature request:

For an external library like glpk, one needs to add all include paths. With bzlmod, the paths to include files are prefixed with external/glpk~VERSION, external/glpk~ or external/glpk+ depending on the Bazel version. There currently isn't any predefined Make variable to get this path.

The current workaround is to add it to includes instead which doesn't need this prefix but this is suboptimal as this include path is propagated to all consumers.

Which category does this issue belong to?

Rules API

What underlying problem are you trying to solve with this feature?

See TODO comment in bazelbuild/bazel-central-registry#3066.

Which operating system are you running Bazel on?

Linux

What is the output of bazel info release?

7.0.2, 7.1, 8.0rc

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse HEAD ?

No response

Have you found anything relevant by searching the web?

No response

Any other information, logs, or outputs that you want to share?

No response