golang: Rename GoPackage/Environment to GO_PKG_VARS

The variable is a list of shell variables; the new name is more in-line
with other parts the build system (CONFIGURE_VARS, MAKE_VARS, etc.).

GoPackage/Environment is kept (for now) in case other feeds are using
it.

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