[core] Change ext-py3 package installation path (#3720)
## What changes were proposed in this pull request?
- Giving old style file path was failing `./build/env/bin/pip freeze` with the following error:
```
harsh.gupta@Harshs-MacBook-Pro hue % ./build/env/bin/pip freeze
ERROR: Exception:
Traceback (most recent call last):
File "/Users/harsh.gupta/workspace/hue/upstream/hue/build/env/lib/python3.8/site-packages/pip/_internal/cli/base_command.py", line 180, in exc_logging_wrapper
status = run_func(*args)
File "/Users/harsh.gupta/workspace/hue/upstream/hue/build/env/lib/python3.8/site-packages/pip/_internal/commands/freeze.py", line 98, in run
for line in freeze(
File "/Users/harsh.gupta/workspace/hue/upstream/hue/build/env/lib/python3.8/site-packages/pip/_internal/operations/freeze.py", line 43, in freeze
req = FrozenRequirement.from_dist(dist)
File "/Users/harsh.gupta/workspace/hue/upstream/hue/build/env/lib/python3.8/site-packages/pip/_internal/operations/freeze.py", line 236, in from_dist
editable = dist.editable
File "/Users/harsh.gupta/workspace/hue/upstream/hue/build/env/lib/python3.8/site-packages/pip/_internal/metadata/base.py", line 338, in editable
return bool(self.editable_project_location)
File "/Users/harsh.gupta/workspace/hue/upstream/hue/build/env/lib/python3.8/site-packages/pip/_internal/metadata/base.py", line 179, in editable_project_location
return url_to_path(direct_url.url)
File "/Users/harsh.gupta/workspace/hue/upstream/hue/build/env/lib/python3.8/site-packages/pip/_internal/utils/urls.py", line 43, in url_to_path
raise ValueError(
ValueError: non-local file URIs are not supported on this platform: 'file://Users/harsh.gupta/workspace/hue/upstream/hue/desktop/core/ext-py3/pysaml2-7.3.1'
```
- Using 2 slash file path is installing the packages + fixes the above error.
ocal setup