[2.2.0] Extra directory in source tree trashes build

Just found that after create some temp/ directory in source tree I was unable to build module. Looks like some bits are missing in pyproject.toml.

Here is test case:

```console

+ mkdir temp

+ /usr/bin/python3 -sBm build -w --no-isolation

* Getting build dependencies for wheel...

error: Multiple top-level packages discovered in a flat-layout: ['github', 'temp'].



To avoid accidental inclusion of unwanted files or directories,

setuptools will not proceed with this build.



If you are trying to create a single distribution with multiple packages

on purpose, you should not rely on automatic discovery.

Instead, consider the following options:



1. set up custom discovery (`find` directive with `include` or `exclude`)

2. use a `src-layout`

3. explicitly set `py_modules` or `packages` with a list of names



To find more information, look for "package discovery" on setuptools docs.



ERROR Backend subprocess exited when trying to invoke get_requires_for_build_wheel

```

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