Rename back to swift package completion-tool (#7600)

Rename `swift package completion-command` back to `swift package
completion-tool`.

### Motivation:

The subcommand seems accidentally renamed to `swift package
completion-command` while renaming the type name in
#7336

### Modifications:

This change renames it back to `swift package completion-tool` to keep
compatibility with the previous version.

This should be cherry-picked to release/6.0 too.

### Result:

Keep command name compatibility

Author: kateinoigakukun

Sources/Commands/PackageCommands/CompletionCommand.swift

@@ -18,6 +18,7 @@ import var TSCBasic.stdoutStream
 extension SwiftPackageCommand {
     struct CompletionCommand: SwiftCommand {
         static let configuration = CommandConfiguration(
+            commandName: "completion-tool",
             abstract: "Completion command (for shell completions)"
         )
 

Tests/CommandsTests/PackageCommandTests.swift

@@ -70,6 +70,11 @@ final class PackageCommandTests: CommandsTestCase {
         XCTAssertMatch(stdout, .contains("Swift Package Manager"))
     }
 
+    func testCompletionTool() throws {
+        let stdout = try execute(["completion-tool", "--help"]).stdout
+        XCTAssertMatch(stdout, .contains("OVERVIEW: Completion command (for shell completions)"))
+    }
+
 	func testInitOverview() throws {
 		let stdout = try execute(["init", "--help"]).stdout
 		XCTAssertMatch(stdout, .contains("OVERVIEW: Initialize a new package"))