Simplify getting data for `datashader_dashboard`

[maximlt]

Getting data for datashader_dashboard is convoluted, it required anaconda-project as a dependency to run prepare on other projects and uses intake to wire everything. I don't think this dependence on other projects is very useful in the end, we should simplify it.

[jbednar]

Agreed. Too messy; even I forget how it works, and I have some vague idea that I might have written it.