Create list of HTML dependencies for D3. Each version has
a distinct root D3 object so it's possible to combine multiple versions
of D3 on a single page. For example, D3 v5 is accessed via d3v5 and
D3 v4 is accessed via d3v4. Note however that D3 v3 is accessed via
simply d3 (for compabibilty with existing htmlwidgets that use
this form).