Magento 2.3 – Reference to Undeclared Plugin

If you use redis, run:

redis-cli flushall

If you also use an environment like ansible (lxc - server), you'll need to do that command in container. Plus I did the following:

• Clear var/generation (or generated/* in newer versions)
• Clear var/cache
• Check if the module generating the error is enabled and enable it (bin/magento module:enagle Some_Module) or Enable all Magento modules: bin/magento module:enable --all
• Compile DI bin/magento setup:di:compile

I couldn't reproduce your error, but I got an other one.

Then I did some minor adjustments to your code and it worked.
First drop the InstallSchema.php file. It does nothing so it is useless.

Then make sure that your plugin file is in Amaj/CorePlugin/Model/Currency/Plugin.php and it looks like this:

<?php

namespace Amaj\CorePlugin\Model\Currency;

class Plugin
{
    protected $logger;

    public function __construct(\Psr\Log\LoggerInterface $logger) {
        $this->logger = $logger;
    }

    /**
     * @param $subject
     * @param \Closure $proceed
     * @param $price
     * @param array $options
     * @return string
     */
    public function aroundFormatTxt(
        $subject,
        \Closure $proceed,
        $price,
        $options = []
    )
    {
        $this->logger->info('Amaj aroundFormatTxt');
        return '2';
    }

}

you don't need parent::__construct(); in the constructor because there is no parent class.
And I replaced $logger->info with $this->logger->info because there is no $logger variable.