cmd/cover: add new "emit meta file" mode for packages without tests
Introduce a new mode of execution for instrumenting packages that have
no test files. Instead of just skipping packages with no test files
(during "go test -cover" runs), the go command will invoke cmd/cover
on the package passing in an option in the config file indicating that
it should emit a coverage meta-data file directly for the package (if
the package has no functions, an empty file is emitted). Note that
this CL doesn't actually wire up this functionality in the Go command,
that will come in a later patch.