[5.8] Init template cleanup (#6144)

[bitjammer]

Improve the swift package init experience using the following criteria:

• Simplify the templates to include only the minimum necessary to get started without making assumptions.
• Use documentation links instead of template content to teach people about Swift.
• Improve the "front door" experience with the help usage.

Detailed list of changes:

• Remove the system-module template.
• Remove the manifest template
• Add a new tool template, an executable that uses ArgumentParser a manifest instead.
• Remove README generation.
• Don't generate empty directories.
• Don't emit empty dependencies array in manifests.
• Update library template content:
  • Remove struct definition, use a simple public function instead.
  • Include documentation links for TSPL and the Standard Library.
  • Include documentation links for XCTest in the generated test case.
• Update executable template content
  • Use simpler top-level code instead of an @main struct.
  • Remove executable tests.
  • Put sources directly in ./Sources.
• Clean up and align type argument help text
• Update Usage.md documentation
• Update CHANGELOG to reflect init template changes in Init template cleanup #6144

rdar://98999734

[bitjammer]

@swift-ci Please smoke test

[neonichu]

C:\Users\swift-ci\jenkins\workspace\swiftpm-PR-windows\swift-tools-support-core\Sources\TSCBasic\FileSystem.swift:14:8: error: module compiled with Swift 5.9 cannot be imported by the Swift 5.8 compiler: T:\6\swift\SystemPackage.swiftmodule

I think this is a known issue on the 5.8 Windows CI? At least it looks familiar...