Add describe changed command to check if a package needs rebuild

[geropl]

Description

Adds leeway describe changed <package> — a lightweight command that checks whether a package's current version hash exists in the local or remote cache.

• Exits 0 if the package is cached (unchanged / no rebuild needed)
• Exits 1 if the package is not cached (changed / needs rebuild)

This enables CI branching decisions without running a full build --dry-run across the entire workspace:

if leeway describe changed my-component:my-package; then
  echo "unchanged, skipping build"
else
  leeway build my-component:my-package
fi

The command reuses the existing LocalCache.Location() and RemoteCache.ExistingPackages() infrastructure, so it respects the same LEEWAY_CACHE_DIR, LEEWAY_REMOTE_CACHE_BUCKET, and LEEWAY_REMOTE_CACHE_STORAGE configuration as leeway build.

Output is a tab-separated line: <package>\t<version>\t<status> where status is one of cached locally, cached remotely, or changed.

Related Issue(s)

Fixes CORE-

How to test

# Build leeway
go build -o leeway .

# Check a package that hasn't been built — should exit 1
./leeway describe changed //:app
echo $?  # 1

# Build a package
./leeway build //:helloworld

# Check again — should exit 0
./leeway describe changed //:helloworld
echo $?  # 0