refactor: remove dependency on Pkg

[SebastianM-C]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

I don't think Pkg is needed anymore in the package. It is only used in check_pkg_version, but that function isn't public API and not used by anything as far as I can tell. If the functionality is still needed, then Base.pkgversion can be used.

help?> Base.pkgversion
  pkgversion(m::Module)

  Return the version of the package that imported module m, or nothing if m was not imported from a package, or imported from a package without a version field set.

  The version is read from the package's Project.toml during package load.

  To get the version of the package that imported the current module the form pkgversion(@__MODULE__) can be used.

  │ Julia 1.9
  │
  │  This function was introduced in Julia 1.9.