This builds on the indexer changes to support test projects using the
tibs build system. See the commit message from IndexStoreDB for more
information.
This commit adds a couple of simple tests using test fixtures in the
INPUTS directory. It is a fairly minimal change for using the indexer's
test support code to prove the model works. One missing piece here is
support for mutable sources and updating the index, which is supported
on the IndexStoreDB side, but needs a bit more work here. This also
doesn't include any tests using swiftpm's build system.
3044f7394b