terraform/schema: Make schema storage version-aware

[radeksimko]

User impact

This doesn't yet fully implement 0.13 support nor does it deal with provider aliasing, but at least provides backward-compatible functionality for 0.12-style configs in 0.13 where provider sources aren't declared explicitly.

Before

After

Implementation details

The new internal/terraform/addrs package is mostly just a copy of these two files:
https://github.com/hashicorp/terraform/blob/master/addrs/provider.go
https://github.com/hashicorp/terraform/blob/master/addrs/provider_test.go

I just omitted functions which LS doesn't need (yet) and changed error handling from tfdiags to simpler multierror.

I intentionally didn't copy the whole package yet, but it may come down it as it's likely one of the ways of resolving #36 reliably.

[ghost]

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