Multiclient in modular support

[qiaozha]

This PR is mainly focus on how to implement multiple subclients in the Modular library and see the design https://github.com/Azure/azure-sdk-for-js/pull/26164/files

And the following items are non-goals of this PR:

1. multiple endpoints scenario in RLC layer. Issue tracked [DPG]Multi-Endpoint support in Modular #1894
2. identify the shared models and only generate it once. See more details in this issue https://github.com/Azure/typespec-azure/issues/3035
3. Split the RLC layer from Modular.

In this packages/typespec-test/test/loadtesting_modular smoke test case, we have two subclients LoadTestRunClient and LoadTestAdministrationClient in the Modular layer, and they both point to the same @service, which means the RLC layer is single endpoint.

our current code structure is

On the subpath exports, we export the following subpaths

   "exports": {
    ".": {
      "types": "./types/src/index.d.ts",
      "require": "./dist/index.cjs",
      "import": "./dist-esm/src/index.js"
    },
    "./loadTestAdministration": {
      "types": "./types/src/loadTestAdministration/index.d.ts",
      "import": "./dist-esm/src/loadTestAdministration/index.js"
    },
    "./loadTestAdministration/api": {
      "types": "./types/src/loadTestAdministration/api/index.d.ts",
      "import": "./dist-esm/src/loadTestAdministration/api/index.js"
    },
    "./loadTestAdministration/models": {
      "types": "./types/src/loadTestAdministration/models/index.d.ts",
      "import": "./dist-esm/src/loadTestAdministration/models/index.js"
    },
    "./loadTestRun": {
      "types": "./types/src/loadTestRun/index.d.ts",
      "import": "./dist-esm/src/loadTestRun/index.js"
    },
    "./loadTestRun/api": {
      "types": "./types/src/loadTestRun/api/index.d.ts",
      "import": "./dist-esm/src/loadTestRun/api/index.js"
    },
    "./loadTestRun/models": {
      "types": "./types/src/loadTestRun/models/index.d.ts",
      "import": "./dist-esm/src/loadTestRun/models/index.js"
    }
  },

Based on our latest discussion, "We only need one way to import these things without there being ambiguity about which way is correct". I have removed the top level api/index.ts and models/index.ts in the multi-client modular case.

Remove ./rest subpath export as discussed in the Q&A session.

[qiaozha]

Still have a few places to improve.

[qiaozha]

Left some comments, hope this can accelerate the review.

[xirzec]

A couple thoughts

[qiaozha]

@xirzec @mpodwysocki I have removed the top level api/index.ts and models/index.ts in the multi clients case. Could you help review again and let me know if you have any comments ? Thanks

[qiaozha]

Pending on #1933 to be fixed.