add-target breaks a package that it's a single target package and doesn't have a subdirectory under Sources

[kkebo]

Is it reproducible with SwiftPM command-line tools: swift build, swift test, swift package etc?

• Confirmed reproduction steps with SwiftPM CLI. The description text must include reproduction steps with either of command-line SwiftPM commands, swift build, swift test, swift package etc.

Description

swift package add-target breaks a package that it's a single target package and doesn't have a subdirectory under Sources.

Expected behavior

The package can be built and its directory hierarchy becomes like this:

.
├── Package.swift
└── Sources
    ├── SamplePackage
    │   └── main.swift
    └── SecondTarget
        └── SecondTarget.swift

Actual behavior

error: 'samplepackage': Source files for target SamplePackage should be located under 'Sources/SamplePackage', or a custom sources path can be set with the 'path' property in Package.swift

.
├── Package.swift
└── Sources
    ├── main.swift
    └── SecondTarget
        └── SecondTarget.swift

Steps to reproduce

mkdir SamplePackage
cd SamplePackage
swift package init --type executable
swift package add-target SecondTarget
swift build

Swift Package Manager version/commit hash

6.0.3

Swift & OS version (output of swift --version ; uname -a)

Swift version 6.0.3 (swift-6.0.3-RELEASE)
Target: aarch64-unknown-linux-gnu
Linux Raspberry-beetle 6.6.74+rpt-rpi-2712 #1 SMP PREEMPT Debian 1:6.6.74-1+rpt1 (2025-01-27) aarch64 GNU/Linux