Use module-style dy_import's rather than `from X import *`

Also, change a `SocketTimeoutError` (from `sockettimeout.r2py` and
thus not in the library's base context anymore) to a plain Repy
`TimeoutError`. The libraries importing `httpretrieve` never seemed
to care about the possibilty of a timeout anyway...
Corrected a dy_import to be module-level (not into context)
Fix a counterproductive default for a kwarg.