check_reqs (`cordova requirements`) treats Xcode different than pod or ios-deploy

# Bug Report

## Problem

### What is expected to happen?

`cordova requirements` should treat all the software it checks for equally and show similar error message for similar problems.

### What does actually happen?

```
λ cordova requirements

Requirements check results for ios:
Apple macOS: installed darwin
Xcode: not installed

ios-deploy: not installed
Cordova needs ios-deploy version 1.9.2 or greater, you have version . Please download, build and install version 1.9.2 or greater from https://github.com/phonegap/ios-deploy into your path, or do 'npm install -g ios-deploy'
CocoaPods: installed 1.0.1
Some of requirements check failed
```

As you can see Xcode/`xcodebuild` only gets a "not installed", while `ios-deploy` also shows a nice error message: "Cordova needs ios-deploy version 1.9.2 or greater, you have version . Please download, build and install version 1.9.2 or greater from https://github.com/phonegap/ios-deploy into your path, or do 'npm install -g ios-deploy'"

When you have a (faked) Xcode installed, the "needs version" error message is correctly displayed:
```
...
Xcode: not installed
Cordova needs xcodebuild version 7.0.0 or greater, you have version 1. Please install version 7.0.0 or greater from App Store
...
```

## Information

This system does _not_ have Xcode/`xcodebuild` and `ios-deploy` installed.

### Command or Code

`cordova requirements`

### Version information

```
λ cordova -v
8.1.2 (cordova-lib@8.1.1)
```

## Checklist

- [x] I searched for existing GitHub issues
- [x] I updated all Cordova tooling to most recent version
- [x] I included all the necessary information above

Start by running `cordova requirements` in an environment without Xcode/`xcodebuild` and `ios-deploy`, then trace the requirements checks that produce their output. Compare the missing-tool paths for Xcode and ios-deploy, and use the existing version-error wording as the expected behavior. Done means both missing tools receive similarly useful version or installation guidance.

Written by the indexing model from the issue text.