Differentiate raw and inferred 1-part source string

[radeksimko]

Hi @hashicorp/terraform-core 👋🏻
This (currently internal, but soon to be published) library is basically a 1-to-1 copy of what's currently in core:

• https://github.com/hashicorp/terraform/blob/main/addrs/provider.go
• https://github.com/hashicorp/terraform/blob/main/addrs/provider_test.go

The only minor change is internal tfdiags package replaced with custom error type. All the diagnostics from the address parser were sourceless anyway.

---

This PR is related to our conversation we had some time ago on Slack about what does ParseProviderSourceString do with 1-part source string, such as aws. In core's case it is parsed as hashicorp/aws.

Because this is IMO highly context-dependent I decided to introduce another parsing function - ParseRawProviderSourceString which instead treats 1-part source string as legacy, i.e. aws is parsed as -/aws.

I also updated the readme to add some context on where and when to use which function.

With all that in mind the old ParseProviderSourceString is the new ParseAndInferProviderSourceString and is intended to be functionally equivalent. As such it should also allow you to adopt this library when it is published even in core.

I'm open to suggestions, especially around the function names.

---

This PR unblocks hashicorp/terraform-schema#35

[alisdair]

This looks good to me. I spotted a typo in a comment, but otherwise ✅