On start.spring.io, if you customize the artifactId it creates a zip file
with the same name. The `spring init` command did not have a similar
shortcut.

This commit updates the request to customize the artifactId if none is
set and a custom location was specified. Just as we check for the
presence of a dot to figure out if we have to extract the archive or not,
we check for it to generate an artifactId without an extension.

In practice, `spring init foo` creates a foo directory with a project
whose artifactId is `foo` and `spring init foo.zip` stores a foo.zip
file with the same project (i.e. the artifactId is `foo`).

Closes gh-3714