[SR-2083] [SwiftPM] Don't error if a directory contains only ignored files

[ddunbar]

Previous ID	SR-2083
Radar	None
Original Reporter	@ddunbar
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Package Manager
Labels	Bug, StarterBug
Assignee	@ddunbar
Priority	Medium

md5: c99ea8a96bda596c30e7cda7c9b28c85

Issue Description:

When switching branches, it is easy for a local directory to end up with stray temporary files. SwiftPM should ignore directories only containing files it would otherwise ignore when trying to issue an error like this:

error: the module at /Volumes/Data/ddunbar/public/swift-project/swiftpm/Tests/Commands does not contain any source files
fix: either remove the module folder, or add a source file to the module
--- bootstrap: error: build failed with exit status 1
$ ls Tests/Commands/
BuildToolTests.swift~   PackageToolTests.swift~ TestToolTests.swift~    XCTestManifests.swift~

[Bouke]

PR: #710