Reconsider how we treat and build the SDK

[sodic]

It seems we simultaneously treat the SDK as a package and as a project:

• Package: we don't run npm install inside its folder, it's installed indirectly through the root package.json)
• Project: We run tsc inside.

I can't remember exactly whether this is the case, but I'm 80% sure. We should double-check before further digging in. If that's the case, devDependencies may behave unexpectedly.

This is wrong and is probably the root cause of the problem described in #2085.

I'm guessing that older versions of NPM treat linked dependencies differently than the newer ones, and that it only works with the newer versions because we're relying on undefined behavior.

To understand what I mean, check the difference between a package-lock.json files generated with NPM versions 0.9.4 and 0.10.x