command/fmt: Make implicit provider requirements explicit

Ever since Terraform v0.13 we've been recommending that module authors be
explicit about which providers their modules are intended to work with,
but we retained some heuristics for detecting undeclared dependencies on
official providers for backward compatibility with modules intended for
Terraform v0.12 and earlier.

In the intervening years we've found that those heuristics have caused a
lot of confusion, because Terraform tends not to be clear about what
guesses it is making but then returns confusing error messages that only
make sense if you are already an expert on Terraform's compatibility
heuristics. These have been some of the most frequently asked questions
about Terraform in community forums over the years.

In an attempt to make these rules easier to understand, "terraform fmt"
will now run the same heuristics and then generate explicit declarations
reflecting what it detected. This makes it explicit that it's good style
to declare your dependencies explicitly, and also gives new users a clearer
signal about what Terraform is guessing so that they have a better chance
of learning what they might need to change if Terraform's guesses were
not correct.