pipx run -> _download_and_run: error branch can never execute properly

[itsayellow]

Describe the bug
In commands.py, in function run(): run() calls _download_and_run() with the variable package_or_url where _download_and_run() is expecting argument package.

The result is that if the if branch executes for this statement in _download_and_run():

    if not (venv.bin_path / app).exists():

_download_and_run() will try and execute venv.get_venv_metadata_for_package(package).

If the package argument to _download_and_run() is in fact a url (as it could be if run() is called with a --spec argument) then venv.get_venv_metadata_for_package(package) will fail because it is expecting a package name and not a url.

How to reproduce
One would need to call pipx run with a --spec argument with a url and a non-existent app. For example by modifying an example from the pipx makefile:
pipx run --spec=git+https://github.com/cs01/nox.git@5ea70723e9e6 notnox

Unexpected behavior
Because notnox is not an app in the package, the error if statement will execute. However venv_metadata_inspector.py will find no package named git+https://github.com/cs01/nox.git@5ea70723e9e6 and so will return empty information about the venv. Thus the printed error message has no apps to show as available or associated with the package in the venv.

[itsayellow]

After merging #222, I cannot reproduce the bug (pipx works fine now.)