Nixpkgs: Easy wins for the `mono` closure size

[delroth]

Splitting dev outputs from some dependencies

libgdiplus does not have a "dev" output, and drags in many development libs unnecessarily: cairo-dev, mesa-dev, etc. That's ~300MB saved.

Re-enabling stripping

dontStrip was added to the mono package in 2010, mentioning that stripping breaks mcs --version. As far as I can tell, this is not true anymore, and from my testing nothing seems to break when stripping. That's another ~300MB saved.

Splitting out the runtime

The mono package currently contains both the mono runtime and a bunch of development tools. In order to reduce closure size it would be nice to see this package split up.

As an example, anyone who installs steam currently gets a transitive dep to a 32 bit python2.7 on their system because of the "mono-heapviz" script.

[aanderse]

Awesome work @delroth!

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[delroth]

1 and 2 are implemented, I'll look at implementing 3 again (and kill some python2 dependencies).