…rib#3572)

The `imports` attribute is treated as target-relative in non-venv mode,
but venv-mode
was passing the value through without the target-relative processing.
This went unnoticed
because the main use case, pypi packages, are a the repo root, so target
and repo
relative paths are the same.

To fix, use join the input value to the target's path, as done in the
non-venv
`get_imports()` function.

Fixes bazel-contrib#3565