Fix Vertex/Anthropic by adding subpath support for provider modules

[jacobmikesell]

Added subpath field to provider model and updated import logic to support subpaths for npm packages. This allows providers like google-vertex-anthropic to properly import from specific subpaths.

Tested with

{
  "$schema": "https://opencode.ai/config.json",
  "model": "google-vertex-anthropic/claude-3-7-sonnet@20250219",
  "provider": {
    "google-vertex-anthropic": {
      "models": {
        "claude-3-7-sonnet@20250219": {},
      },
      "options": {
        "project": "my project",
        "location": "my location"
      }
    }
  }
}