install_extension crashes due to attempt to mkdir an empty path (v1.8.0)

[Rongronggg9]

When I did python3 setup.py build_ext for breezy, I got the below error:

Running `rustc --crate-name brz --edition=2021 breezy/main.rs --tons-of-flags`
Finished dev [unoptimized + debuginfo] target(s) in 3m 16s
error: [Errno 2] No such file or directory: ''

However, if I locked setuptools-rust==1.7.0, everything went fine:

Running `rustc --crate-name brz --edition=2021 breezy/main.rs --tons-of-flags`
Finished dev [unoptimized + debuginfo] target(s) in 3m 18s
Copying rust artifact from target/debug/brz to brz
# Cython logs ...

I guess the regression was introduced by 7ced8d2

setuptools-rust/setuptools_rust/build.py

Lines 378 to 385 in 7ced8d2

	cwd = os.getcwd()
	if dylib_path.startswith(cwd):
	dylib_path = os.path.relpath(dylib_path, cwd)
	if ext_path.startswith(cwd):
	ext_path = os.path.relpath(ext_path, cwd)
	os.makedirs(os.path.dirname(ext_path), exist_ok=True)
	logger.info("Copying rust artifact from %s to %s", dylib_path, ext_path)

Let's assume:

cwd = '/<<cwd>>'
ext_path = '/<<cwd>>/executable'

The above code snippet will resulting in:

>>> ext_path = os.path.relpath(ext_path, cwd)
>>> assert ext_path == 'executable'
>>> assert os.path.dirname(ext_path) == ''
>>> os.makedirs('', exist_ok=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<frozen os>", line 225, in makedirs
FileNotFoundError: [Errno 2] No such file or directory: ''