virtualenv: determine 'site-packages' based on implementation name

[BurntSushi]

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

[charliermarsh]

I filed #2095 with some more info on what I think a "proper" fix would look like.