update!: moved all program elements in sub packages to root

[sttk]

This PR removes sub packages and moves program element in them to the root package.

The reason for this modification is as follows:

• The sub package name errors conflicts the one of go standard package.
• Not only the root package but also its sub-packages must be imported.

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NOTE: This PR was reset and force-pushed manually.

[Copilot]

Pull Request Overview

This PR consolidates the cliargs package by removing sub-packages and moving all program elements to the root package. The main motivation is to avoid naming conflicts with Go's standard errors package and simplify the import structure by eliminating the need to import multiple sub-packages.

Key changes:

• Moved all error types from errors sub-package to root cliargs package
• Moved validator functions from validators sub-package to root cliargs package
• Updated all import statements and type references throughout the codebase

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
validators_test.go	Updated package name and import path from validators sub-package to cliargs
validators.go	Moved from validators sub-package to root, updated error type references
parse_test.go	Removed errors import, updated error type references to cliargs namespace
parse.go	Removed errors import, updated error type instantiation to cliargs namespace
parse-with_test.go	Removed sub-package imports, updated error and validator type references
parse-with.go	Removed errors import, updated error type instantiation to cliargs namespace
parse-for_test.go	Removed errors import alias, updated error type references to cliargs namespace
parse-for.go	Removed errors import, updated error type instantiation to cliargs namespace
example_errors_test.go	Updated package name and import, changed error alias usage
errors_test.go	Updated package name and import from errors sub-package to cliargs
errors.go	Moved from errors sub-package to root, removed package comment