Remove github.com/google/shlex dependency 🧹 💨

[cthach]

The github.com/google/shlex module was archived on Dec 29, 2022. It is only used once. The github.com/mattn/go-shellwords module is a drop-in replacement and we already import it.

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Manual Test Plan

Test Environment

Used the following tsh config at ~/.tsh/config/config.yaml:

aliases:
  "l": "tsh login --proxy=teleport.dev:443 --user=teleport-admin"

Test Cases

List the various tests that were run to validate the change. Cases should be as exhaustive as possible.

• Running ./build/tsh l should resolve the alias.