The generator does not read Gir.toml from dependencies, missing out on
trait renames like `ObjectExt` turning into GstObjectExt` (to prevent
clashes with `glib`'s `ObjectExt`). Renaming through `trait_name` is
now taken into account in `gir` thanks to [1], but the renames still
need to be availble to all crates referencing this type. Fortunately
only `Gst.Object` is affected - other renamed traits are not extended in
any of the other crates.
[1]: https://github.com/gtk-rs/gir/pull/1108
The previous commit reinstantiated `girs_dir`, and this commit applies
the rename to `girs_directories` and adds the new/missing
`gst-gir-files` directory where GStreamer-specific `.gir` files live.
This reverts commit 7f9fcb09e2.
`generator.py` is in the process of being moved to a generic place in
the `gir` repository for reuse across crates. This means `-d` cannot be
passed for our GStreamer-specific `gst-gir-files` directory anymore, and
should be configured from `Gir.toml` instead.
We were already using `gir -d` and especially now that our files are
separated across two directories that are relative to the directory
containing Gir.toml this only becomes cumbersome. Besides `gir` lacks
functionality to normalize the path, leading to ie.
gstreamer-gl/egl/sys/../../../gir-files in the version comment as a
result.
