Support project SDK

[rauanmayemir]

When working on monorepos, it's possible to use Modules in IDEA, that gives some level of isolation to subprojects. You can load and unload its source code from the IDE, allowing to free up some memory.

As plz spans across all the subprojects, it would make sense to have a global project that will be on the global scope and have full visibility, leaving modules as platform or team-specific workspaces.

I think it would be akin to Bazel projects with the root at .plzconfig.