fix: Use a "package" htmlDependency for local copy of leaflet-providers

[gadenbuie]

Updates providers_default() and underlying functions to return an HTML dependency that points to the packaged file directly, rather than a temporary copy.

pkgload::load_all()
#> ℹ Loading leaflet.providers

providers_default()$dep |> str()
#> List of 10
#>  $ name      : chr "leaflet-providers"
#>  $ version   : chr "2.0.0"
#>  $ src       :List of 1
#>   ..$ file: chr "leaflet-providers"
#>  $ meta      : NULL
#>  $ script    : chr "leaflet-providers.js"
#>  $ stylesheet: NULL
#>  $ head      : NULL
#>  $ attachment: NULL
#>  $ package   : chr "leaflet.providers"
#>  $ all_files : logi FALSE
#>  - attr(*, "class")= chr "html_dependency"