Feature Request: include_file() or subfile()

I'd like to be able to do something like

```
subfile('versions.meson')
subfile('package-defines.meson')
subfile('opengl.meson')
subfile('options.meson')
subfile('linkerflags.meson')
```

to easily share the same snippets across multiple modules/projects without having to copy'n'paste them.

This could be done via subdirectories with subdir() of course, but that seems awkward, and I'd like to keep all those files together in one directory. I think this is nicer.

Start by examining the existing subdir() behavior and the handling of Meson files, then determine how a subfile() or include_file() operation would share snippets across modules and projects. Done should provide a defined, tested way to reuse files from one directory without copying them into each project.

Written by the indexing model from the issue text.