Fix newly added clippy warnings

[taiki-e]

warning: use of `or_insert_with` to construct default value
   --> lcov2cobertura/src/lib.rs:351:22
    |
351 |                     .or_insert_with(Package::default)
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `or_default()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unwrap_or_default
    = note: `#[warn(clippy::unwrap_or_default)]` on by default

warning: accessing first element with `args.files.get(0)`
  --> src/main.rs:68:15
   |
68 |     } else if args.files.get(0) == Some(&PathBuf::from("-")) {
   |               ^^^^^^^^^^^^^^^^^ help: try: `args.files.first()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first
   = note: `#[warn(clippy::get_first)]` on by default

warning: accessing first element with `args
                     .files.get(0)`
  --> src/main.rs:75:24
   |
75 |           let filename = args
   |  ________________________^
76 | |             .files
77 | |             .get(0)
   | |___________________^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first
help: try
   |
75 ~         let filename = args
76 +             .files.first()
   |

[mike-kfed]

thank you!