Preparation tracking needs to be per file rather than per target

[bnbarham]

We currently track preparation per target with the assumption that a change in a target only causes dependents to be re-prepared, not the target itself. This is mostly true for our existing build systems, but isn't in the more general case:

• Plugins could depend on a configuration file within the target and then generate source. If we don't re-prepare after a change to the configuration file, we'd miss updating source.
• For mixed language targets (which we don't have in SwiftPM today), any .swift change must re-prepare so that the generated header is rebuilt.

We should instead track preparation separately for the files (or file types?) of a target.

As an extension we could also either:

1. Generate fake targets in the build graph for plugin generation (which targets could then depend on so that they are re-prepared on input file changes)
2. Add the input files of the plugin command to the files for the target they're generating files for (with some flag to mark them as such)
3. Give generated files an extra input files field

[ahoppen]

Synced to Apple’s issue tracker as rdar://142670165