Add `packages-install-path` in dbt_project

[vchetyrkine]

First of all, thanks for this package, it's very useful :)

Using it on a project recently where we wanted to specify where the deps would be downloaded, we saw that small part was missing from the json schema (packages-install-path).

• [x] I did sort the json keys

[vchetyrkine]

Sorry if this is not the correct move, but I guess I can tag @joellabes for a review ? :)

[vchetyrkine]

No worries, I was not in a hurry ! Thanks 🤗