Make the path argument to `cargo dylint upgrade` optional

[smoelius]

Make this optional:

$ cargo dylint upgrade --help
Upgrade the library package at <PATH> to the latest version of `clippy_utils`

Usage: cargo-dylint dylint upgrade [OPTIONS] <PATH>
                                             ^^^^^^

Default to the current directory when PATH is not provide.

The changes that would be required are the following:

• Change path's type from String to Option<String> in two places:
  • https://github.com/trailofbits/dylint/blob/d75ae91f34613a329a231797a0142066194b5f3d/cargo-dylint/src/main.rs#L128
  • https://github.com/trailofbits/dylint/blob/d75ae91f34613a329a231797a0142066194b5f3d/dylint/src/opts.rs#L110
• Handle the case when path is None here: https://github.com/trailofbits/dylint/blob/d75ae91f34613a329a231797a0142066194b5f3d/dylint/src/package_options/mod.rs#L100