Bundler should be able to use a `.gemspec` file in the absence of Gemfile

Describe the problem as clearly as you can

Often times a Gemfile is not needed for development but other people are accustomed to using it/bundler. In such cases a dummy Gemfile with the contents

gemspec

is sufficient.

Since the Gemfile doesn't add anything except another file, bundler should fall back to a .gemspec file if a Gemfile is not found and treat it like the above dummy file. The fallback should only work if there is a single .gemspec file (or, even more strict, a single ${cur_dir}.gemspec file, e.g. mygem/mygem.gemspec).

The issue names no files or tests. Start by tracing Bundler's Gemfile discovery behavior and its handling of a single .gemspec; verify the fallback works only when no Gemfile exists and exactly one eligible .gemspec is present, while existing behavior remains unchanged for multiple gemspecs.