3.3.1: create temp directory in source tree causes build fail

After create temp/ directory in source tree I found that build fails.
Looks like some bits are missing in pyproject.toml

Here is test case

```console
+ mkdir temp
+ cd jaraco.classes-3.3.1
+ ASMFLAGS='-m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -O2 -g -grecord-gcc-switches -pipe -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -fdata-sections -ffunction-sections -fstack-protector-strong -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -flto=auto -Wp,-U_FORTIFY_SOURCE,-D_FORTIFY_SOURCE=3 -Wp,-D_GLIBCXX_ASSERTIONS -Wall -Werror=format-security'
+ CFLAGS='-m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -O2 -g -grecord-gcc-switches -pipe -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -fdata-sections -ffunction-sections -fstack-protector-strong -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -flto=auto -Wp,-U_FORTIFY_SOURCE,-D_FORTIFY_SOURCE=3 -Wp,-D_GLIBCXX_ASSERTIONS -Wall -Werror=format-security'
+ CXXFLAGS='-m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -O2 -g -grecord-gcc-switches -pipe -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -fdata-sections -ffunction-sections -fstack-protector-strong -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -flto=auto -Wp,-U_FORTIFY_SOURCE,-D_FORTIFY_SOURCE=3 -Wp,-D_GLIBCXX_ASSERTIONS -Wall -Werror=format-security'
+ FFLAGS='-m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -O2 -g -grecord-gcc-switches -pipe -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -fdata-sections -ffunction-sections -fstack-protector-strong -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -flto=auto -Wp,-U_FORTIFY_SOURCE,-D_FORTIFY_SOURCE=3 -Wp,-D_GLIBCXX_ASSERTIONS -Wall -Werror=format-security -I/usr/lib64/gfortran/modules'
+ FCFLAGS='-m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -O2 -g -grecord-gcc-switches -pipe -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -fdata-sections -ffunction-sections -fstack-protector-strong -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -flto=auto -Wp,-U_FORTIFY_SOURCE,-D_FORTIFY_SOURCE=3 -Wp,-D_GLIBCXX_ASSERTIONS -Wall -Werror=format-security -I/usr/lib64/gfortran/modules'
+ LDFLAGS='-specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -Wl,--gc-sections -Wl,--as-needed -Wl,--build-id=sha1 -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld -Wl,-z,pack-relative-relocs -flto=auto -fuse-linker-plugin'
+ RUSTFLAGS='-C codegen-units=1 -C debuginfo=2 -C opt-level=2 -C link-arg=-Wl,--as-needed -C link-arg=-Wl,--build-id=sha1 -C link-arg=-Wl,-z,now -C link-arg=-specs=/usr/lib/rpm/redhat/redhat-hardened-ld -C link-arg=-Wl,-z,pack-relative-relocs -C link-arg=-Wl,-z,relro -C link-arg=-flto=auto --cap-lints=warn'
+ VALAFLAGS=-g
+ CC=/usr/bin/gcc
+ CXX=/usr/bin/g++
+ FC=/usr/bin/gfortran
+ AR=/usr/bin/gcc-ar
+ NM=/usr/bin/gcc-nm
+ RANLIB=/usr/bin/gcc-ranlib
+ export ASMFLAGS CFLAGS CXXFLAGS FFLAGS FCFLAGS LDFLAGS VALAFLAGS CC CXX FC AR NM RANLIB RUSTFLAGS VALAFLAGS
+ PBR_VERSION=3.3.1
+ PDM_BUILD_SCM_VERSION=3.3.1
+ SETUPTOOLS_SCM_PRETEND_VERSION=3.3.1
+ /usr/bin/python3 -sBm build -w --no-isolation
* Getting build dependencies for wheel...
error: Multiple top-level packages discovered in a flat-layout: ['jaraco', '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
```

Start by inspecting pyproject.toml and reproduce the failure by creating a temp/ directory, then run /usr/bin/python3 -sBm build -w --no-isolation. Check the package-discovery configuration against the reported setuptools error; done means the build completes successfully with the extra directory present.

Written by the indexing model from the issue text.