earlydecoder: avoid crash by validating provider names

[radeksimko]

This is to address hashicorp/terraform-ls#1026

[dbanck]

This fixes the crash for me! 👍

But I never see the diagnostics published to VS Code which might be another bug?

[radeksimko]

But I never see the diagnostics published to VS Code which might be another bug?

It is true that we never publish the diagnostics from earlydecoder, although we store them, so that dependent jobs can check those. We could publish them, but most of these diagnostics here can be little vague - so that would need to be addressed first.

There's also a question of duplication between some future diagnostics we end up producing from the main (version & schema specific) decoder + HCL diagnostics. So I'm not against publishing these diags, but I think it should be considered in the wider context in hashicorp/vscode-terraform#720