golang: fix package clean / autoremove, add output whitespace

golang: fix package clean / autoremove, add output whitespace

go 1.11 added modules, which are cached locally. The go developers have
decided to make this cache read-only (golang/go#27455), which causes
problems with package clean / autoremove (#7635).

This adds a call to clear this cache right after building, as currently
there is no easy way to hook into autoremove (it may be possible to hook
into package clean).

This also adds whitespace (blank lines) to certain places in make
output, to aid debugging.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>