Can't load shared libraries from Virtualenv built binaries

Nobody has claimed this yet.

I have the following in my `pyoxidizer.bzl` file:

```
# Package things from a populated virtualenv.
packaging_rules.append(Virtualenv(path="../build/blah/venv"))
```

where `blah` is a project built using custom tooling. Lots of the dependencies of `blah` include shared libraries (e.g. `_psutil_osx`). But because the venv packages were *not* built by PyOxidizer, the `distutils` hack was never invoked and therefore we have no static libraries.

The resulting binary will exit on the `ImportError` but it might be possible to do *something* to support these, even if it's not as efficient as statically linking.

In another project, we wrote a small finder that can do this. The details probably won't be exactly the same, but it will give you an idea of what can be done:

```
class SOFinder:
def __init__(self, so_map, cache_root, archive):
self.so_map = so_map
self.cache_root = cache_root
self.archive = archive

def find_spec(self, name, paths, module=None):
origin = self.so_map.get(name)
if origin is None:
return None

extracted_path = Path(self.cache_root) / origin
if not extracted_path.exists():
self.archive.extract(origin, self.cache_root)

loader = ExtensionFileLoader(name, extracted_path)

return ModuleSpec(name, loader, origin=extracted_path)
```

Where `so_map` was a dict of so names to paths within a zip file.

Start with the Virtualenv(path="../build/blah/venv") packaging rule in pyoxidizer.bzl and trace how shared-library extensions are handled when the virtualenv was built externally. Compare that flow with the provided SOFinder example. Done means binaries built from such virtualenvs can import extensions such as _psutil_osx without requiring statically linked libraries.

Written by the indexing model from the issue text.