Use vanity urls to change the go module name

**Describe the problem/challenge you have**
Details in https://github.com/carvel-dev/carvel/issues/689

**Describe the solution you'd like**
Use vanity urls to change the go module name in kapp (use imgpkg [PR](https://github.com/carvel-dev/imgpkg/pull/597) as a reference)

**Anything else you would like to add:**

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