internal/filesystem: Integrate spf13/afero

[radeksimko]

This (slightly bigger) PR has a few motivating factors:

1. It's becoming clear that the language server will need to read files not only from the memory (as maintained by language client via LSP), but also from the OS filesystem. A module in Terraform is represented by *.tf files in a directory and we can't expect that all relevant files in the directory are opened by the client.
   This complexity deserves a clearer separation between raw filesystem API and the "document API" (per LSP's naming convention) where document is a file which holds some additional metadata the language server needs to access to comply with the protocol.
2. We will eventually need to integrate with at least some parts of the parser currently living in Terraform core and much of the parser there relies on afero.Fs, which makes it a natural choice for the language server as well.
3. This sets a foundation which later allows us to add another layer (OS filesystem) in such a way that reading will be attempted from in-memory FS first and OS FS second, to comply with the LSP.
   Relevant WIP is in b46ec44 and https://github.com/hashicorp/terraform-config-inspect/pull/49/files

This is just an initial refactoring PR which integrates with afero's in-memory filesystem and should have no effect on the end-user.

Related:

• Support provider aliases and 0.13 identities #164

[appilon]

This was a doozy, but LGTM

[ghost]

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