delete the replace domain in go.mod for go install

[SparkYuan]

Package terraform contains codes about "terraform/dag" and "terraform/tfdiags" copied from Terraform.

Terraform is an open-source project but makes almost all codes in an internal package. It's not a friendly way to benefit the open-source community.
The latest non-internal Terraform version is v0.15.3, but the k8s related modules in it are less than v1.15. That is too old for us
and is incompatible with our existing modules.

We import terraform/dag to reuse its DAG structure, and we will build our own DAG structure to support more complex Kusion workflow later.
We import terraform/tfdiags because terraform/dag imported this module, this module will be deleted together when we replace the DAG
structure with our own.

I update go version to 1.18 because of generics are used in TF's codes

[coveralls]

Pull Request Test Coverage Report for Build 3656276777

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 72.381%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/kusionctl/cmd/cmd.go	0	1	0.0%

Totals
Change from base Build 3645101894:	0.4%
Covered Lines:	4581
Relevant Lines:	6329

---

💛 - Coveralls

[howieyuen]

directly copy Terraform's source code???

[SparkYuan]

directly copy Terraform's source code???

Yes, because of the limitation of Terraform, currently, we have to reuse some code by copying source code instead of importing by go mod.

We will upgrade our DAG structure and delete these TF related code in Jan 2023

[howieyuen]

please rebase main branch and re-arrange these commits as follows:

1. replace TF module with 3rd-party
2. fix uts
3. upgrade go verison to 1.18, golinter to 1.50+
4. gofumpt codes

[howieyuen]

delete this file: third_party/pulumi/doc.go