Fix false positives for SPM packages using custom path: argument

[swizzlr]

Problem:
The tool produced false positives for SPM packages that use custom path: arguments in their target definitions. When Sources/TargetName didn't exist, it fell back to scanning ALL of Sources/ and attributed every file to that target.

Example: A package with:
.target(name: "MyModule", path: "Sources/Core")
.target(name: "MyModuleUI", path: "Sources/UI")

Would look for Sources/MyModule, not find it, fall back to Sources/, scan all files, and report UI code as violations for MyModule.

Solution:

1. Added path: String? property to SwiftPackageTarget to store custom paths from target definitions

2. Updated PackageSwiftFileVisitor to parse the path: argument from SPM target function calls using SwiftSyntax

3. Modified PackagesParser to:

   • Use the explicit path from Package.swift when available
   • Normalize paths (strip leading/trailing slashes) to prevent double-slash issues
   • Throw customPathNotFound error if custom path doesn't exist (prevents silent false positives from scanning zero files)
   • Only fall back to convention (Sources/TargetName) when no custom path is specified

Tests:

• Added CustomPathPackage fixture with path: "Sources/Core" for target
• Added Sources/UI/ with undeclared import to verify files outside custom path are NOT scanned (negative test case)
• Added testTarget with custom path (CustomTests/) to verify .testTarget() works correctly with path: argument
• Added InvalidCustomPathPackage fixture to test customPathNotFound error

🤖 Generated with Claude Code

[michaelversus]

Thank you for your contribution @swizzlr !

[codecov-commenter]

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[swizzlr]

No worries, thanks for taking the time to review!