v2: npm-managed extension behavior questions

[boneskull]

Currently in Appium 2 (and #16443), appium <driver|plugin> install <extension> does not write a new dependency to any package.json.

It may make sense to actually write the dependency, if the closest package.json contains appium as a dependency. I cannot decide if this would violate the element of least surprise (assuming we give notice to the end-user).

A potential issue is that when appium installs an extension, it will do so with --global-style (in #16443), which prevents deduping. If a user deletes node_modules, then does an npm install again, the resulting node_modules will be deduped, and thus will not be identical to the directory hierarchy after appium was used.

Perhaps makes sense to just not use --global-style in the case that we have an appium dep in package.json, and let npm do whatever it's going to do. Once extensions start getting installed via other means than appium, we will be losing control; we cannot feasibly rectify the situation at runtime without detection of a deduped module tree and re-installation via --global-style.

This does not imply that we shouldn't be using --global-style if APPIUM_HOME is, say, ~/.appium; I think that's the right choice.

[jlipps]

Yeah, I'm kind of thinking the whole "use NPM to manage dependencies" is an "I'm using Node and I know what I'm doing" category. So it would make sense to not use --global-style in that case. We should probably make clear that you should only use NPM yourself or use the Appium CLI, but not both.