Support initializing build directory from existing workspace
In cases the source code already resides in a workspace-like structure, the configure phase tries to create another workspace, which ends up in a multilevel
builddir/src/package_name/src constructs that compiler fails to import and build successfully.
This patch adds another environment variable
DH_GOLANG_EXISTING_WORKSPACE, which if set, prevents creating the workspace into
$builddir. It will copy all the workspace structure from existing project directory instead.