I am trying to put up my first php project on packagist.org but my vendor name is already taken. I have already pushed a tagged release to github; now I have to modify my composer file without creating a new git tag which isn't as simple as just pushing your changes to github. My question is: how do I do a namespace check before naming the package? Same issue with the code name spacing too. How can I do a lookup to prevent a clash?
You can do a check using search:
composer search somevendor
Then just wait to see if some packages pop up.
You can't really do a check for namespacing though. I guess your best bet is sticking with your vendor name. And even if you have a colliding vendor name, your project name will usually still differ.