The change is required for unit testing the logic.

…root

Fix: #3312

Previously, when packages added in `in_app_include` were installed
to a location outside of the project root directory, span from
those packages were not extended with OTel compatible source code
information. Cases include running Python from virtualenv created
outside of the project root directory or Python packages installed into
the system using package managers. This resulted in an inconsistency:
spans from the same project would be different, depending on the
deployment method.

In this change, the logic was slightly changed to avoid these
discrepancies and conform to the requirements, described in the PR with
better setting of in-app in stack frames:
#1894 (comment).