Bindings not picked up from dependency <command>

0install doesn't seem to be picking up bindings from <command>
dependencies. This seems like a bug, although it's possible commands
aren't intended to be used for what I'm trying to do.

Specifically, I provide <command>s which aren't really commands, but
are more just extra bindings. As an example:

http://gfxmonk.net/dist/0install/edit-server.xml

contains (right down the bottom):

    <command name="systemd-unit" path="edit-server">
      <environment insert="xdg/config" mode="prepend" name="XDG_CONFIG_DIRS"/>
      <runner interface="http://repo.roscidus.com/python/python"/>
    </command>

The intent is not to run this directly, but to depend on it as
<requires command="systemd-unit" .... />, and you'll inherit the
appropriate environment variables to make this service available in a
user systemd session.

While the XDG_CONFIG_DIRS binding is applied when running a command
directly, running a feed which depends on this command causes the
command-specific binding to be ignored:

$ 0install run --wrapper='env #' --command systemd-unit
http://gfxmonk.net/dist/0install/edit-server.xml | grep XDG_CONFIG
XDG_CONFIG_DIRS=/home/tim/.cache/0install.net/implementations/sha256new_XVAFKEINOAZS2BZN3CL2G2AMWZ3LO5CGG2FY2FTMH7HHSNEOWAOA/xdg/config:/etc/xdg

$ touch intentionally_blank
$ cat tmp.xml
<?xml version="1.0" encoding="utf-8"?>
<?xml-stylesheet type='text/xsl' href='interface.xsl'?>
<interface xmlns="http://zero-install.sourceforge.net/2004/injector/interface">
    <name>test</name>
    <summary>test</summary>
    <description>
    </description>

    <group>
        <requires
interface="http://gfxmonk.net/dist/0install/edit-server.xml"
command="systemd-unit"/>
        <command name="run" path="intentionally_blank"/>
    <implementation id="." version="1"/></group>

</interface>

$ 0install run --wrapper='env #' tmp.xml | grep XDG_CONFIG
(no output)



If this is a feature (rather than a bug), is there some way to provide
a set of opt-in bindings by name, rather than requiring the user write
these themselves? I've always been a little uncomfortable with
requiring users to write bindings which depend on the directory
structure of my implementation, but I can also see this being very
important for opt-in feature sets which export a handful of
environment variables (which becomes very fragile if you need to
duplicate multiple bundings in every feed which requires the given
feature).

Cheers,
 - Tim.

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