Libraries path is hardcoded to `/vendor`

[julpar]

On Deprecation#L129 it assumes vendor dir is always /vendor [1]. As per composer config [2] that behaviour can be changed. In a project i'm working on it happen to be this way:

    "config": {
        "vendor-dir": "libraries"
    },

Any chance that path can be fetched from config or specified on Deprecation::enableWithTriggerError();?

[1] `https://github.com/doctrine/deprecations/blob/d12b33583aa1d591d8c45d31812df46d8d19e579/lib/Doctrine/Deprecations/Deprecation.php#L129

[2] https://getcomposer.org/doc/06-config.md#vendor-dir

[greg0ire]

Related comment: https://github.com/doctrine/deprecations/pull/21#discussion_r592725942