Bump swift-tools-version to 5.5 (#696)

In addition to Swift Concurrency, Swift 5.5 also introduces `get throws` properties, which would allow us to fix numerous warnings related to non-throwing `AbsolutePath.init` deprecation.

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.3
+// swift-tools-version:5.5
 
 import PackageDescription
 
@@ -28,7 +28,7 @@ let package = Package(
       // See 'Dependencies' below.
     ],
     targets: [
-      .target(
+      .executableTarget(
         name: "sourcekit-lsp",
         dependencies: [
           "LanguageServerProtocolJSONRPC",
@@ -65,7 +65,6 @@ let package = Package(
           "LSPTestSupport",
           "SourceKitLSP",
           .product(name: "ISDBTestSupport", package: "IndexStoreDB"),
-          .product(name: "tibs", package: "IndexStoreDB"), // Never imported, needed at runtime
           .product(name: "SwiftToolsSupport-auto", package: "swift-tools-support-core"),
         ], 
         resources: [

Sources/sourcekit-lsp/SourceKitLSP.swift

@@ -54,7 +54,8 @@ extension PathPrefixMapping: ExpressibleByArgument {
 extension LogLevel: ExpressibleByArgument {}
 extension BuildConfiguration: ExpressibleByArgument {}
 
-struct Main: ParsableCommand {
+@main
+struct SourceKitLSP: ParsableCommand {
   static let configuration = CommandConfiguration(
     abstract: "Language Server Protocol implementation for Swift and C-based languages"
   )
@@ -209,5 +210,3 @@ struct Main: ParsableCommand {
     dispatchMain()
   }
 }
-
-Main.main()