go-mod.bbclass: Add class for `go mod` support

When using Go Modules, the the current working directory MUST be at or
below the location of the 'go.mod' file when the go tool is used, and
there is no way to tell it to look elsewhere.  It will automatically
look upwards for the file, but not downwards.

To support this use case, we provide the `GO_WORKDIR` variable, which
defaults to `GO_IMPORT` but allows for easy override.

[YOCTO #13883]

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>