Bug#973462: ITP: podcasts -- Podcast application for GNOME written in Rust

[Diego Escalante Urrelo <diegoe@gnome.org>]

Package: wnpp
Followup-For: Bug #973462
X-Debbugs-Cc: diegoe@gnome.org

I'm working on the GNOME stack side of things, with a bunch of updated
crates already:
 https://salsa.debian.org/marvil07/debcargo-conf/-/commits/diegoe_gnome-stack

At the very least, these should be updated and fine:
    glib-macros: package 0.10.1
    proc-macro-crate: package 0.1.5
    pangocairo-sys: update to 0.11.0
    gtk-sys: update to 0.10.0
    gdk-sys: update to 0.10.0
    cairo-sys-rs: update to 0.10.0
    gdk-pixbuf-sys: update to 0.10.0
    gio-sys: update to 0.10.1
    winapi: update to 0.3.9
    atk-sys: update to 0.10.0
    pango-sys: update to 0.10.0
    gobject-sys: update to 0.10.0
    glib-sys: update to 0.10.1

Of course, it's all pointless unless the full chain of deps is updated,
but here's where I ran into problems:

I'm currently stuck trying to update futures-*, because futures-task
does not build its corresponding +feature packages when `debcargo` is
run with `--config` (which is the case when using the `debcargo-conf`
scripts), this stops me from updating other futures-* crates.

It seems that debcargo behaves differently when a config override is
passed, as the Provides: list and a few other things change:

With --config (...)/debcargo.toml:
```
    Package: librust-futures-task-dev
    Architecture: any
    Multi-Arch: same
    Depends:
     ${misc:Depends},
     librust-once-cell-1+default-dev (>= 1.3.1-~~)
    Provides:
     librust-futures-task+default-dev (= ${binary:Version}),
     librust-futures-task+std-dev (= ${binary:Version}),
     librust-futures-task-0-dev (= ${binary:Version}),
     librust-futures-task-0+default-dev (= ${binary:Version}),
     librust-futures-task-0+std-dev (= ${binary:Version}),
     librust-futures-task-0.3-dev (= ${binary:Version}),
     librust-futures-task-0.3+default-dev (= ${binary:Version}),
     librust-futures-task-0.3+std-dev (= ${binary:Version}),
     librust-futures-task-0.3.7-dev (= ${binary:Version}),
     librust-futures-task-0.3.7+default-dev (= ${binary:Version}),
     librust-futures-task-0.3.7+std-dev (= ${binary:Version})
```

Without --config:
```
    Package: librust-futures-task-dev
    Architecture: any
    Multi-Arch: same
    Depends:
     ${misc:Depends}
    Recommends:
     librust-futures-task+std-dev (= ${binary:Version})
    Suggests:
     librust-futures-task+once-cell-dev (= ${binary:Version})
    Provides:
     librust-futures-task+alloc-dev (= ${binary:Version}),
     librust-futures-task+cfg-target-has-atomic-dev (= ${binary:Version}),
     librust-futures-task+unstable-dev (= ${binary:Version}),
     librust-futures-task-0-dev (= ${binary:Version}),
     librust-futures-task-0+alloc-dev (= ${binary:Version}),
     librust-futures-task-0+cfg-target-has-atomic-dev (= ${binary:Version}),
     librust-futures-task-0+unstable-dev (= ${binary:Version}),
     librust-futures-task-0.3-dev (= ${binary:Version}),
     librust-futures-task-0.3+alloc-dev (= ${binary:Version}),
     librust-futures-task-0.3+cfg-target-has-atomic-dev (= ${binary:Version}),
     librust-futures-task-0.3+unstable-dev (= ${binary:Version}),
     librust-futures-task-0.3.7-dev (= ${binary:Version}),
     librust-futures-task-0.3.7+alloc-dev (= ${binary:Version}),
     librust-futures-task-0.3.7+cfg-target-has-atomic-dev (= ${binary:Version}),
     librust-futures-task-0.3.7+unstable-dev (= ${binary:Version})
```

I'm still learning about rust, so I'm clueless about what's going on
here, or if it's even a bug in debcargo (since this only happens when
passed a --config option).