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:

  • dylint/cargo-dylint/src/main.rs

    Line 128 in d75ae91

    path: String,

  • dylint/dylint/src/opts.rs

    Line 110 in d75ae91

    pub path: String,

• Handle the case when path is None here:

  dylint/dylint/src/package_options/mod.rs

  Line 100 in d75ae91

  let path = Path::new(&upgrade_opts.path);