Fix potential infinite recursion in TerraformImportCommand

[duckpuppy]

Fixes #439

Merge Checklist

• Have you linked this Pull Request to an Issue?
• Have you updated any relevant README files, to explain how this new feature works (with examples)?
• Have you added relevant test cases for your change?
• Do all tests and code style checks pass with ./gradlew check --info?
• Have you successfully run your change in a pipeline in a Jenkins instance?
• Have you updated the CHANGELOG?

[codecov-commenter]

Codecov Report

Merging #440 (42eed27) into master (20765fb) will not change coverage.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master     #440   +/-   ##
=======================================
  Coverage   82.49%   82.49%           
=======================================
  Files          54       54           
  Lines        1508     1508           
  Branches      321      321           
=======================================
  Hits         1244     1244           
  Misses        115      115           
  Partials      149      149           

Impacted Files	Coverage Δ
src/TerraformImportCommand.groovy	85.29% <66.66%> (ø)

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[kmanning]

There's documentation for this feature - https://github.com/manheim/terraform-pipeline/blob/master/docs/TerraformImportPlugin.md

1. Is the documentation correct?
2. Does the example from the documentation reproduce the error?
3. Were you able to reproduce the error? (Ie: do we know that this will fix the problem)
4. Can we add the documentation to the landing README, if this is a feature that is "ready"?