Opaque error message when xcodebuild is unavailable

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Describe the bug

If xcodebuild -version returns

xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance

swiftlint fails with

$ swiftlint lint
Fatal error: Loading sourcekitd.framework/Versions/A/sourcekitd failed: file SourceKittenFramework/library_wrapper.swift, line 31
[1]    83461 trace trap  swiftlint lint

Could we add some kind of detection for this condition and emit a more useful error message?

Complete output when running SwiftLint, including the stack trace and command used

Crash report:
swiftlint_2021-03-10-100023_H2WF637UQ6NY.crash.zip

Environment

• SwiftLint version (run swiftlint version to be sure)? 0.43.0
• Installation method used (Homebrew, CocoaPods, building from source, etc)? Homebrew
• Paste your configuration file:

See https://github.com/mapbox/mapbox-maps-ios/tree/412981f0712d5f76d6ce41015d21e9fdb9cdb521

• Are you using nested configurations?
  If so, paste their relative paths and respective contents.

Yes:

$ find . -name ".swiftlint.yml" 
./.swiftlint.yml
./Tests/.swiftlint.yml
./Sources/.swiftlint.yml

• Which Xcode version are you using (check xcodebuild -version)?

xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance

• Do you have a sample that shows the issue? Run echo "[string here]" | swiftlint lint --no-cache --use-stdin --enable-all-rules
  to quickly test if your example is really demonstrating the issue. If your example is more
  complex, you can use swiftlint lint --path [file here] --no-cache --enable-all-rules.

No

Start with the swiftlint lint command and SourceKittenFramework/library_wrapper.swift, using the reported xcodebuild -version failure as the reproduction case. Add detection for the unavailable Xcode condition and verify that SwiftLint emits a useful error instead of the reported crash.