[psr-4] Move /packages directory to /src under single Rector\\ namespace

[TomasVotruba]

Follow up to #5411

composer.json Before

{
    "autoload": {   
        "psr-4": {
             "Rector\\": ["rules",  "packages"],
             "Rector\\Core\\": "src"
        }
    }
},
{
    "autoload-dev": {   
        "psr-4": {
             "Rector\\Tests\\": ["rules-tests", "packages-tests"],
             "Rector\\Core\\Tests\\": "tests"
        }
    }
}

composer.json After

{
    "autoload": {   
        "psr-4": {
             "Rector\\": ["rules", "src"]
        }
    }
},
{
    "autoload-dev": {   
        "psr-4": {
             "Rector\\": ["rules-tests", "tests"]
        }
    }
}