Move validators to pub_validation package and install via dart install

[mosuem]

Description

This PR moves in-tree package validators out of package:pub into a standalone package package:pub_validation.

Behavior on dart pub publish

1. Workspace Dependency Detection:
   • If pub_validation (or the configured validation package) is declared in the package's pubspec.yaml (dev_dependencies, dependencies, or dependency_overrides), pub runs the locally resolved version using dart run <package>:<package>. This allows developers to pin, customize, or fork validator packages safely via audited workspace dependencies without fetching code at publish time.
2. Fallback to Global Installation:
   • If not declared in pubspec.yaml, pub installs the default version via dart install pub_validation@^0.1.0-dev and executes the binary from Dart's global install directory.
3. Structured Diagnostics:
   • pub_validation executes with --json on the package directory, and pub parses reported errors, warnings, and hints to present to the user.

Configuration & CLI Options

• --validation-package=<package[@descriptor]>: Specifies the validation package to install and run (defaults to pub_validation@^0.1.0-dev).
• --validation-hosted-url=<url>: Specifies the package server from which to install the validation package (defaults to https://pub.dev).