For what it is worth, the Sigil epubcheck plugin downloads as EpubCheck_v0.4.6.3.zip. After it is downloaded and installed in Sigil, the first run will locate and download the latest version of epubcheck (currently 5.0.1). The current Sigil plugin version does not include the epubcheck.jar and other files due to the size of the download (the plugin .zip file clocks in at ~51KB while the epubcheck.zip clocks in at ~29MB.
Download the Sigil epubcheck plugin from the first message in the
epubcheck plugin for Sigil thread.