Prefix target names with 'GraphViz' to avoid collisions

[elliottwilliams]

Target names must be globally unique, even once SE-0026 is fully implemented (see https://forums.swift.org/t/multiple-target-issue-with-spm/16696/4). The "Core" target added in #15 introduces a naming conflict with XcodeGen, and while I've proposed XcodeGen rename their "Core" target (yonaskolb/XcodeGen#1057) it probably makes sense to rename the target here as well, to avoid similar conflicts in the future.

The naming convention is obviously subject to bikeshedding – lmk if there's a different way you'd like to organize these :)

For examples of other packages that have had to do things like this, see apple/swift-argument-parser#74 and apple/swift-argument-parser@d8fb6bd

[mattt]

Thanks for pointing this out, @elliottwilliams. Sorry for any hassle this may have caused you.

I think the whole "multiple internal modules" approach is poorly motivated, so I decided to simply do away with it in #18. Please let me know if that causes any problems.