参考：
https://blog.csdn.net/zhenghongzhi6/article/details/107895535
https://forum.unity.com/threads/visibility-changes-for-preview-packages-in-2020-1.910880/
以下插件不再在 Package Manager可见，需要点击红框处处理，输入其包名，点击Add按钮
- com.ptc.vuforia.engine
- com.unity.2d.entities
- com.unity.ai.planner
- com.unity.aovrecorder
- com.unity.assetbundlebrowser
- com.unity.assetgraph
- com.unity.barracuda
- com.unity.barracuda.burst
- com.unity.build-report-inspector
- com.unity.cloud.userreporting
- com.unity.collections
- com.unity.connect.share
- com.unity.dots.editor
- com.unity.entities
- com.unity.film-tv.toolbox
- com.unity.google.resonance.audio
- com.unity.immediate-window
- com.unity.mathematics
- com.unity.meshsync
- com.unity.multiplayer-hlapi
- com.unity.package-manager-doctools
- com.unity.package-manager-ui
- com.unity.package-validation-suite
- com.unity.physics
- com.unity.platforms
- com.unity.platforms.android
- com.unity.platforms.linux
- com.unity.platforms.macos
- com.unity.platforms.web
- com.unity.platforms.windows
- com.unity.playablegraph-visualizer
- com.unity.render-pipelines.lightweight
- com.unity.rendering.hybrid
- com.unity.renderstreaming
- com.unity.scene-template
- com.unity.simulation.client
- com.unity.simulation.core
- com.unity.simulation.capture
- com.unity.simulation.games
- com.unity.standardevents
- com.unity.streaming-image-sequence
- com.unity.test-framework.performance
- com.unity.tiny.all
- com.unity.transport
- com.unity.upm.develop
- com.unity.vectorgraphics
- com.unity.webrtc
- com.unity.xr.googlevr.android
- com.unity.xr.googlevr.ios
- com.unity.xr.legacyinputhelpers
- com.unity.xr.oculus.android
- com.unity.xr.oculus.standalone
- com.unity.xr.openvr.standalone
- com.unity.xr.arsubsystems
- com.unity.xr.interactionsubsystems
- com.unity.xr.windowsmr.metro
