Preserve leading "./" in paths for Debian package compatibility

[vrmiguel]

Hi @charliermarsh, congrats on the project!

This PR suggests fixing Header::set_path's 'incorrect' behavior of stripping away the leading dot-slash in paths.
The reasoning for this is that Debian/Ubuntu packaging (.debs) require their data tarballs to state their entries as follows:

drwxr-xr-x root/root         0 2023-07-29 15:58 ./
drwxr-xr-x root/root         0 2023-07-29 15:58 ./usr/
drwxr-xr-x root/root         0 2023-07-29 15:58 ./usr/lib/
drwxr-xr-x root/root         0 2023-07-29 15:58 ./usr/lib/postgresql/
drwxr-xr-x root/root         0 2023-07-29 15:58 ./usr/lib/postgresql/15/
drwxr-xr-x root/root         0 2023-07-29 15:58 ./usr/lib/postgresql/15/lib/
drwxr-xr-x root/root         0 2023-07-29 15:58 ./usr/lib/postgresql/15/lib/bitcode/
drwxr-xr-x root/root         0 2023-07-29 15:58 ./usr/lib/postgresql/15/lib/bitcode/vector/
drwxr-xr-x root/root         0 2023-07-29 15:58 ./usr/lib/postgresql/15/lib/bitcode/vector/src/
-rw-r--r-- root/root     20460 2023-07-29 15:58 ./usr/lib/postgresql/15/lib/bitcode/vector/src/ivfbuild.bc

The current tar-rs processing instead converts those into:

drwxr-xr-x root/root         0 2023-07-29 15:58 ./
drwxr-xr-x root/root         0 2023-07-29 15:58 usr/
drwxr-xr-x root/root         0 2023-07-29 15:58 usr/lib/
drwxr-xr-x root/root         0 2023-07-29 15:58 usr/lib/postgresql/
drwxr-xr-x root/root         0 2023-07-29 15:58 usr/lib/postgresql/15/
drwxr-xr-x root/root         0 2023-07-29 15:58 usr/lib/postgresql/15/lib/
drwxr-xr-x root/root         0 2023-07-29 15:58 usr/lib/postgresql/15/lib/bitcode/
drwxr-xr-x root/root         0 2023-07-29 15:58 usr/lib/postgresql/15/lib/bitcode/vector/
drwxr-xr-x root/root         0 2023-07-29 15:58 usr/lib/postgresql/15/lib/bitcode/vector/src/
-rw-r--r-- root/root     20460 2023-07-29 15:58 usr/lib/postgresql/15/lib/bitcode/vector/src/ivfbuild.bc

The lack of leading dot-slash in the files above makes Debian tooling (apt/dpkg) to extract files in the incorrect folders, or reject the .deb entirely (now I can't recall for sure)

This has been a known tar-rs issue for years, described in issues such as alexcrichton/tar-rs#263 and alexcrichton/tar-rs#335, but we haven't had much luck in hearing back from Alex, so I suppose it's better to try to fix this here, downstream