Support 0.13 provider identities

[radeksimko]

This PR closes the loop on 0.13 support and closes #164

Implementation details

• A separate simplified parser (terraform-config-inspect) is used to parse the provider identities. This might however in the near future be replaced by a dedicated version-aware parser. The aim here is just to deliver 0.13 support early and avoid the extra pressure there would be on delivering the new parser.
• It is becoming more obvious that we'll need a stateful parser, or that we'll need to maintain the parsed AST in which we can lookup provider identities, variables, outputs, etc. etc. The existing parser is not well designed for that as it is stateless and parses configs lazily. This PR doesn't solve the problem in the most ideal way, since it introduces additional parser, therefore the same unique config will be parsed twice. It's a trade-off for bringing 0.13 support early.
• We (re)parse the each module on initialize so we can match providers correctly. This in turn means we spend more CPU and memory during the initial discovery phase. This in turn puts more pressure to resolving Set a cap for auto-loading root modules #186
• Provider aliases generally do not affect what schema will be available, there will always be at most 1 version of a provider (as identified by its identity) and mapping of provider names to identities is done in terraform.required_providers. This cannot be overridden by aliases. Aliases only affect configuration differences, not schema. This is why the parser can ignore them at this stage.
• Changes were introduced to the Filesystem interface to make it compatible with tfconfig.FS and to reflect that files may be changed by client, but not saved to disk and the in-memory copy needs to be prioritized as per LSP.
  • Allow parsing module from any filesystem terraform-config-inspect#49 introduces the relevant FS interface to upstream in terraform-config-inspect in a backwards-compatible way
  • in theory all FS operations could use the new FS interface, but the PR is only introducing it to the parser as that's where it is critical and also well implemented/configured client should only be sending updates for *.tf files, not module manifests nor plugin lock files - so these will be read from the disk in the foreseeable future anyway.

[appilon]

LGTM

[ghost]

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