Welcome to WordPress hell. We have 2018 and WordPress still does not have any dependency management and still didn’t notice Composer existence.
WordPress ecosystem simply relies on assumption, that functions/classes names of plugins/themes should be unique. Obviously distributing popular 3rd-part Composer libraries with your plugin/theme is asking for trouble – it is easy to get names collision when other plugin is doing the same. There is no good way out of this situation.
If you want a bulletproof solution for standalone plugins, you should prefix namespaces of every package in your
vendor directory with plugin prefix, for example
myplugin\vendors\GuzzleHttp\Client, so there is no risk of name collisions. This will require some work to write script for this (or you may use some existing solutions, like
humbug/php-scoper), and you may get many duplicated dependencies (10 plugins may bring the same library 10 times, but with different namespaces), but this is the cost of integrating modern tools and patterns into outdated software.
If you’re writing this plugin for yourself and you’re controlling final installation, you may try to use Composer for installing WordPress and its plugins. You still may need to fix 3rd-party plugins (via forking) if they’re have bundled some composer library, but in the long run it should simplify many things and you may avoid duplicating libraries for every plugin.