virtualenv: determine 'site-packages' based on implementation name
I'm not at all sure whether this is a correct fix or not, but it does
seem to make `pypy` work in at least some cases with `uv`. Previously,
I couldn't get it to work at all. Namely the virtualenv was created
with a `lib/python3.10/site-packages`, but whenever I did a `uv
pip install` in that virtualenv, it was looking for a non-existent
`lib/pypy3.10/site-packages` directory.
With this PR, the workflow reported as not working in #1488 now works
for me:
```
$ pypy3 --version
Python 3.10.13 (fc59e61cfbff, Jan 17 2024, 05:35:45)
[PyPy 7.3.15 with GCC 13.2.1 20230801]
$ uv venv --python $(which pypy3) --seed
Using Python 3.10.13 interpreter at: /usr/bin/pypy3
Creating virtualenv at: .venv
+ pip==24.0
+ setuptools==69.1.1
+ wheel==0.42.0
Activate with: source .venv/bin/activate
$ uv pip install 'alembic==1.0.11'
Resolved 9 packages in 8ms
Installed 9 packages in 14ms
+ alembic==1.0.11
+ greenlet==3.0.3
+ mako==1.3.2
+ markupsafe==2.1.5
+ python-dateutil==2.8.2
+ python-editor==1.0.4
+ six==1.16.0
+ sqlalchemy==2.0.27
+ typing-extensions==4.10.0
```
Where as previously (current `main`), I was hitting this error:
```
$ uv venv --python $(which pypy3) --seed
Using Python 3.10.13 interpreter at: /usr/bin/pypy3
Creating virtualenv at: .venv
+ pip==24.0
+ setuptools==69.1.1
+ wheel==0.42.0
Activate with: source .venv/bin/activate
$ uv pip install 'alembic==1.0.11'
error: Failed to list installed packages
Caused by: failed to read directory `/home/andrew/astral/issues/uv/i1488/.venv/lib/pypy3.10/site-packages`
Caused by: No such file or directory (os error 2)
```
Notice though that neither outcome above matches the error reported in #1488,
so this is likely not a complete fix. There are perhaps other lurking
issues.
Ref #1488
