Tikz library `ducks` is not portable

[muzimuzhi]

Currently, when ducks is loaded as a tikz libray, it loads tikzlibraryducks.code.tex, which loads tikzducks.sty through \usepackage{tikzducks}. Since \usepackage is a LaTeX command, this makes the tikz library ducks unusable in other formats, hence not portable.

https://github.com/samcarter/tikzducks/blob/d5ebc8365c094af1173dba006fc9dc7b1ee957a3/tikzlibraryducks.code.tex#L14