Remove current directory from PATH in PEP 517 hooks (#1975)
## Summary
When you invoke `python -c`, an empty string is prepended to `sys.path`,
which allows loading modules in the current directory
(https://docs.python.org/3/using/cmdline.html#cmdoption-P). However, in
PEP 517 builds, the current directory should _not_ be part of the path.
There's a flag we can use to disable this behavior (`-P`), but it's only
available in Python 3.11 and later, so instead, I'm doing something
similar to pip's `__main__.py`, which avoids this for `python -m pip`
invocations.
Closes https://github.com/astral-sh/uv/issues/1972.
