ability to register file system input dependencies during the configure phase

[andrewrk]

Extracted from #20580.

One pattern in build.zig scripts is to walk a directory during the configure phase and create steps based on the set of files found, perhaps including the file contents found as well. Since it's only possible to create steps during the configure phase, it means the configure phase must be rerun if any files observed this way are added, removed, or modified.

So, the build system API needs a way for build scripts to declare that they have introduced a logical dependency on a particular file system path.

The file system watch mechanism will then respond to detected changes to these files by exiting the build runner process with a special notification to the parent process that it needs to be rerun for exactly this reason.

Related:

• make zig build --watch detect modifications to the build script and rerun itself #20602