Error building swift-format

[hanckmann]

This might be related to the older item #132
When trying to build swift-format I get the following error:

`
λ ~/Projects/swift-format/ format swift build
warning: target 'CCommonMark' in package 'swift-format' contains no valid source files
warning: invalid duplicate target dependency declaration 'swift-build' in target 'FunctionalPerformanceTests'
warning: invalid duplicate target dependency declaration 'swift-package' in target 'FunctionalPerformanceTests'
/Users/patrick/Projects/swift-format/Sources/CommonMark/CMarkInterop.swift:13:8: error: no such module 'CCommonMark'
import CCommonMark
^
/Users/patrick/Projects/swift-format/Sources/CommonMark/CMarkInterop.swift:13:8: error: no such module 'CCommonMark'
import CCommonMark
^
/Users/patrick/Projects/swift-format/Sources/CommonMark/CMarkInterop.swift:13:8: error: no such module 'CCommonMark'
import CCommonMark
^
/Users/patrick/Projects/swift-format/Sources/CommonMark/CMarkInterop.swift:13:8: error: no such module 'CCommonMark'
import CCommonMark
^

`

[hanckmann]

I build via:

$ git checkout format
$ git submodule update --init
$ swift build