fix(Npm): Stop creating dangling packages for non-workspace projects

According to the code comment directly above, adding the packages is
necessary only for projects using workspaces, see also [1]. Executing
the logic is not necessary for non-workspace projects and may lead to
non-referenced packages in the result, for example when scope excludes
are used. Fix that by executing the code only conditionally.

Note: The workspaces support for Pnpm and Yarn currently represents
workspace submodules as non-referenced packages. This change is a
preparation for fixing the workspace submodule representation.

Fixes #9195.

[1]: https://github.com/oss-review-toolkit/ort/pull/4034/files#diff-28d89c724dd945fe93dec5700d61d4dcdaab02f492e09613c8fee6ac5c76cbe6R363-R366

Signed-off-by: Frank Viernau <[email protected]>