Virtualenv-based objdir detection doesn't work for instrumented builds,
as instrumentation-specific objdirs aren't created.
Resolve this by:
- Removing redundant cwd-based topsrcdir resolution - the __file__-based code path below covers this case.
- Only using virtualenv-based resolving if topsrcdir isn't resolved.
- Documenting that virtualenv-based resolution is problematic for instrumented builds.