Build WasmKit with host SwiftPM when SwiftPM is not built

Currently, passing `--wasmkit` to `build-script` also requires `--swiftpm --install-swiftpm` and all of their dependencies (at the very least Dispatch, Foundation, XCTest, Testing, llbuild, swift-build). We can conditionally switch to host SwiftPM in this case.

Author: MaxDesiatov

utils/swift_build_support/swift_build_support/products/wasmkit.py

@@ -73,11 +73,10 @@ def cli_file_path(cls, build_dir):
 
 
 def run_swift_build(host_target, product, swiftpm_package_product_name, set_installation_rpath=False):
-    if product.args.build_runtime_with_host_compiler:
-      swift_build = product.toolchain.swift_build
-    else:
-      # Building with the freshly-built SwiftPM
-      swift_build = os.path.join(product.install_toolchain_path(host_target), "bin", "swift-build")
+    swift_build = os.path.join(product.install_toolchain_path(host_target), "bin", "swift-build")
+
+    if not os.path.exists(swift_build) or product.args.build_runtime_with_host_compiler:
+        swift_build = product.toolchain.swift_build
 
     if host_target.startswith('macos'):
         # Universal binary on macOS