Possibility to depend on the tarball generated by `ninja dist`

I am trying to build RPMs for the current project with a custom target roughly looking like this:
```
spec = configure_file(input: 'foo.spec.in', output: 'foo.spec', configuration: ...)

custom_target('rpm',
input: [spec, tar],
output: 'foo.src.rpm', # there might be other RPMs generated...
command: [
find_program('rpmbuild'),
'--define', '_sourcedir ' + meson.current_build_dir(),
'--define', '_builddir ' + meson.current_build_dir(),
'--define', '_srcrpmdir ' + meson.current_build_dir(),
'--define', '_rpmdir ' + meson.current_build_dir(),
'--clean',
'--ba', '@INPUT0@',
]
console: true,
)
```
However, the problem is the `tar` input. I could try to come up with my own custom target to create the tarfile, but this is cumbersome, and meson already has a means of generating a dist tarfile. Would it be possible to expose this somehow so it can be used as input/dependencies for targets like the one above?

Start by tracing how `ninja dist` produces its tarball and how `custom_target` handles inputs and dependencies. Determine an interface that exposes the generated archive to another target, then validate it with the RPM example using `rpmbuild` and confirm the tarball is built before the RPM target runs.

Written by the indexing model from the issue text.