refactor(shorebird_cli): move executable wrapper classes to src/executables directory

[bryanoltman]

Description

Small organizational change to group our executable wrapper classes in one directory.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (7dda562) 100.00% compared to head (552ae36) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1355   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          152       152           
  Lines         4434      4434           
=========================================
  Hits          4434      4434           

Flag	Coverage Δ
shorebird_cli	100.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...shorebird_cli/lib/src/commands/doctor_command.dart	100.00% <ø> (ø)
...s/shorebird_cli/lib/src/commands/init_command.dart	100.00% <ø> (ø)
...horebird_cli/lib/src/commands/preview_command.dart	100.00% <ø> (ø)
...ackages/shorebird_cli/lib/src/executables/adb.dart	100.00% <ø> (ø)
.../shorebird_cli/lib/src/executables/bundletool.dart	100.00% <ø> (ø)
...ackages/shorebird_cli/lib/src/executables/git.dart	100.00% <ø> (ø)
...ges/shorebird_cli/lib/src/executables/gradlew.dart	100.00% <ø> (ø)
.../shorebird_cli/lib/src/executables/ios_deploy.dart	100.00% <ø> (ø)
...ckages/shorebird_cli/lib/src/executables/java.dart	100.00% <ø> (ø)
.../shorebird_cli/lib/src/executables/xcodebuild.dart	100.00% <ø> (ø)
... and 3 more

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