Enhance `would_clobber()` with more help getting rid of the offending file

[jennybc]

would_clobber() could potentially do more in both interactive and non-interactive sessions:

• Could use a cli hyperlink for runnable code to delete the file that's blocking things. (It would not be automatically runnable, but would still get some nice affordances to make it easily runnable.)
• Could even offer to delete the file, in an interactive session. But just doing the first point is probably enough.

Inspired by #435