Allow scanning sub dir within a larger search context

[wagoodman]

What would you like to be added:
We have multiple issues that want to be able to search within a small space, but reference things outside of that space:

• Support generating sbom for specific pnpm workspace packages #2574
• Support scanning filesystems without building an index #3145 (for performance reasons)
• Maven versions still blank in syft output when using specific search context #3207

It would be nice to allow for something like this:

syft ./my/dir/project-1 --reference ./my/dir

Were I only want to catalog packages within ./my/dir/project-1 but I want to be able to reference file system material from a specific parent directory ./my/dir

In this way if there is a pom.xml in the project dir but the parent pom has required properties, we can reference that material to get the correct version. This applies to any ecosystem where manifests can reference other manifests in parent directories for correct resolution.

Today we have the --exclude flag to tailor the search space, but this doesn't help in a large set of cases (and may result in cumbersome number of flags or brittle configuration).