[#67] Remove the `cmd` parameter from `Tool`, and `cliEnvSettingsRequiredTools` from `CliEnvSettings`

[german1608]

Resolves #67

• Removes the cmd parameter from Tool (and consequently, ToolSelector)
• Add the need function, which does pretty much this: https://github.com/chshersh/iris/blob/main/src/Iris/Env.hs#L123-L126

Additional tasks

• [ ] Documentation for changes provided/changed

  (no docs added since I'm unsure of the final signature of need)

• [x] Tests added
• [ ] Updated CHANGELOG.md

  (should I do this?)