main: new global option -chdir

[apparentlymart]

This new option is intended to address the previous inconsistencies where some older subcommands supported partially changing the target directory (where Terraform would use the new directory inconsistently) where newer commands did not support that override at all.

Instead, now Terraform will accept a -chdir command at the start of the command line (before the subcommand) and will interpret it as a request to direct all actions that would normally be taken in the current working directory into the target directory instead. This is similar to options offered by some other similar tools, such as the -C option in make.

terraform -chdir=some/subdirectory apply

The new option is only accepted at the start of the command line (before the subcommand) as a way to reflect that it is a global command (not specific to a particular subcommand) and that it takes effect before executing the subcommand. This also means it'll be forced to appear before any other command-specific arguments that take file paths, which hopefully communicates that those other arguments are interpreted relative to the overridden path.

As a measure of pragmatism for existing uses, the path.cwd object in the Terraform language will continue to return the original working directory (ignoring -chdir), in case that is important in some exceptional workflows. The path.root object gives the root module directory, which will always match the overridden working directory unless the user simultaneously uses one of the legacy directory override arguments, which is not a pattern we intend to support in the long run.

As a first step down the deprecation path, this commit adjusts the documentation to de-emphasize the inconsistent old command line arguments, including specific guidance on what to use instead for the main three workflow commands, but all of those options remain supported in the same way as they were before. In a later commit we'll make those arguments produce a visible deprecation warning in Terraform's output, and then in an even later commit we'll remove them entirely so that -chdir is the
single supported way to run Terraform from a directory other than the one containing the root module configuration.

---

This will not be merged at least until the main branch opens for 0.14 development. Even then, this is just a first draft and is likely to see some tweaks as we try out some different permutations and see how less common cases are behaving under this new model.

[codecov]

Codecov Report

Merging #26087 into master will decrease coverage by 0.02%.
The diff coverage is 46.15%.

Impacted Files	Coverage Δ
commands.go	1.02% <0.00%> (-0.01%)	⬇️
terraform/context.go	86.46% <ø> (ø)
terraform/evaluate.go	52.33% <33.33%> (-0.73%)	⬇️
main.go	34.70% <41.02%> (+0.64%)	⬆️
command/meta.go	69.19% <100.00%> (+0.13%)	⬆️
help.go	78.00% <100.00%> (+3.00%)	⬆️
configs/hcl2shim/values.go	47.95% <0.00%> (-34.50%)	⬇️
terraform/resource.go	47.14% <0.00%> (-3.63%)	⬇️
terraform/node_resource_plan.go	91.80% <0.00%> (-1.64%)	⬇️
backend/remote-state/http/backend.go	82.17% <0.00%> (-0.67%)	⬇️
... and 7 more

[mildwonkey]

🎉 🎉 🎉 Yay!

[ghost]

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