resolves svelte from node entrypoint & add support for overriding svelte location - fixes #109

[rixo]

Uses require.main.require to resolve svelte from app entrypoint. This will use app's own svelte even when using a symlinked svelte-loader.

The last commit additionaly allows to provide svelte location directly with an env variable. This can be required by some external tools targeting svelte-loader, but have their own entrypoint outside of the main app (full disclosure: I need it for svelte-hmr-spec).

[benmccann]

@rixo is this still helpful to you if we merge the svelte-loader-hot stuff in here? If so, this PR will need to be rebased

[rixo]

I guess it would still be useful but, in hindsight, I've learnt this was a recurring problem when developing multiple linked packages locally. Nowadays I would rather try to address this by hacking the Svelte dependency from the outside, with something like proxyquire, in the test environment that needs it. I can't figure a precise case, but I've got a persistent feeling that the suggested approach could backfire in some other situations.

Closing for now. Will open an updated PR if the need arises again and it really can't be addressed by more targeted solutions.