npm link doesn't play nicely with Sass imports

[shawnbot]

It's currently not possible to npm link this repo into a project that imports our Sass files without adding src/ after @primer/css in the import path. For instance:

@import "@primer/css/support/index.scss";

This works when you npm install @primer/css because we copy src/* up a level in script/prepublish. But if you npm link path/to/primer/css, that script doesn't run, support/index.scss doesn't exist because it lives in src/, and you have to change it your import path:

@import "@primer/css/src/support/index.scss";

This is not great. We have options, though!

1. Add node_modules/@primer/css/src to Sass import paths

   This is the simplest option and involves only updating our docs, but it's also kind of an ugly hack.

2. Move src/* up to / in git

   This fixes the issue, but clutters the repo root with lots of directories, some of which contain SCSS files while others (pages, script, bin, lib, etc.) don't. I prefer to silo off the style sources from other types of files, but I'm open to it if this doesn't bother anyone else.

3. Symlink src/* to / on installation

   We might be able to detect one npm's automatic environment variables to determine if we're running "locally" and add those files to .gitignore so that they're not committed accidentally. I tested npm link on my machine and it doesn't look like we can hook into it with a link script in package.json. But install runs automatically as part of linking, so we might be able to do it there.

   ⚠️ If we're going to do this, I really want us to be diligent about not relying on those files existing in development. We might want to remove the symlinks before tests are run to guard against false positives when resolving Sass import paths, for instance.

4. Sass custom importer sorcery?

   Nope nope nopity nope. Don't even think about it.

[shawnbot]

Another potential gotcha that I just ran into in github/github, and which is pretty specific to that app, is that if you run npm link with a different version of Node than you've last run npm install in this repo, it'll fail because the node-sass native binaries (which are Node version-specific) won't match. (npm runs the prepare script automatically as part of install).

The simplest fix is just to rm -rf node_modules in this repo first. The more "correct" solution is probably not to run dist in our prepare script, as in #718.