doc-generator - allow Oem schema doc be generated

[lindappwu]

Oem schema definitions cannot be generated by doc-generator because Oem object is fungible and doesn't contain any direct references to specific Oem extensions, so the tool doesn't pick up those definitions.

Take an example from "public-oem-examples" with below config.

/doc-generator/sample_inputs/subset/config.json

"import_from": [ "./mockups/public-oem-examples/Contoso.com" ]

/doc-generator/sample_inputs/subset/docgen-subset.json

     "IncludeSchemas": {
        "ContosoServiceRoot": {},
        "ContosoAccountService": {},
        "ContosoTurboencabulatorService": {}
     }

ContosoTurboencabulatorService schema can be generated successfully, but ContosoServiceRoot and ContosoAccountService schema cannot.

Suggest to enhance the tool for including Oem schema definitions.