Add a check for similar names

[martindemello]

Currently, pypi will reject a new package upload if the name is "too similar" to an existing package. However, that is the only place I could find to run this similarity check. Running a search for the name I wanted to use did not bring up the similarly-named package in the first five pages of results; I finally had to guess what the similar name might be and search for it directly.

It would be useful to have a "check if name is valid" search box on the website, where I could have run the search and seen exactly what package was blocking my desired name.

See also #11139