Avoid using $_ when loading the commands from namespaces

[bbrtj]

Summary

Solution proposed in #2175

Motivation

Polluting $_ can happen deep down in dependencies tree of the loaded command.

References

resolves #2175

[Copilot]

Pull Request Overview

This PR refactors the command-loading loop to avoid using the implicit $_, preventing unintended global variable pollution.

• Replaced a chained grep/for using $_ with an explicit for my $pkg loop.
• Added an early next unless _command($pkg) guard.
• Updated the hash assignment to use the named $pkg variable.

Comments suppressed due to low confidence (3)

lib/Mojolicious/Commands.pm:68

• [nitpick] Consider renaming $pkg to a more descriptive name like $module or $command_class to clarify that it represents a command class.

for my $pkg (find_modules($ns), find_packages($ns)) {

lib/Mojolicious/Commands.pm:67

• [nitpick] Add a comment explaining that this loop avoids using $_ to prevent pollution from upstream code, clarifying the purpose of the refactoring.

for my $ns (@{$self->namespaces}) {

lib/Mojolicious/Commands.pm:71

• Consider adding a test case to verify that command loading no longer pollutes $_, ensuring this transformation addresses the original issue (Commands error when topic variable set to undef in compilation #2175).

}