(rfc) recommend installing a plugin when needed

[DeeJay]

Can I urge caution in the area of dependencies? This might be a particular
concern for Windows users working via the 'standalone' installer, but will
probably affect other platforms too.

Some current Windows examples include -

1] graph-ancestry from bzrtools.

after 'fixing' the redundant import of saxutils the plugin will then report
that it needs the Graphviz package. (And through no fault of the
bzr team, on my system the Graphviz installer did not put the
related binaries on my path, so even though they were 'installed' the
graph-ancestry plugin could not find them.)


2] the bzrgtk collection

attempting to run any command from this collection results in
ERROR: PyGTK not installed.

Googling for 'PyGTK on Windows' suggests that it is a twisty
path of further dependencies. There was promise of an installer,
but its (single) host site seems to be down or the file has
been removed.

So in summary - guiding the user to install the right plugin
seems an 'obvious' thing to do, but in two significant cases
it will not on its own be sufficient to allow the user to get useful
results.

DJ