Do not contribute "r.sourceCurrentFile" affordances in an R package

[juliasilge]

Addresses #1359

I can't think of good reasons why people would in fact want any of these affordances in an R package. Even without them, you can still do something like Cmd+Enter to get a function defined, as well as of course the better/recommended options like devtools::load_all(), etc.

QA Notes

After this change,

• when you are in an .R file but not in an R package, you get the command to source your current file on the editor action bar, in the context menu, and via the keybinding.
• when you are in an R package in an .R file, you do not see this command in any of these three spots.

I kept the command in the command palette because it is used in testing (the testing workspaces look like R packages) and it's not super prominent for users there.

[jmcphers]

I think this makes sense for R files that are in the R package, but I'm somewhat concerned people will expect to have this affordance for other R scripts in the workspace (scratch files, utility scripts, etc.) Let's see how it feels.

[juliasilge]

For future work, we could make a new context key that applies only to files in the /R directory in an R package and use that instead.

I do think it's likely having the console and Cmd+Enter gets us where we need to be for how most users approach their scratch/iterative work.