Provide a way for install scripts to access target-specific installation information

To change properties on an installed file, you currently have to do:

```meson
meson.add_install_script('install-script.sh',
get_option('libdir'),
some_build_target.full_path())
```

Which is a bit clumsy. We should provide a way for install scripts to introspect the installation information of targets without having to manually call `mesonintrospect`.

Start with the meson.add_install_script call and the target.full_path() and get_option('libdir') values shown in the issue; compare this flow with mesonintrospect. Define how an install script will access target-specific installation information without manually invoking mesonintrospect. Done means the requested introspection path is available to install scripts.

Written by the indexing model from the issue text.