Fix "add" adding transitive dependencies with "undefined" versions.

[BillyONeal]

Before:

PS C:\Dev\test> type .\vcpkg-configuration.json
{
  "default-registry": {
    "kind": "git",
    "baseline": "e2667a41fc2fc578474e9521d7eb90b769569c83",
    "repository": "https://github.com/microsoft/vcpkg"
  },
  "registries": [
    {
      "kind": "artifact",
      "location": "https://aka.ms/vcpkg-ce-default",
      "name": "microsoft"
    }
  ],
  "requires": {
  }
}
PS C:\Dev\test> C:\Dev\vcpkg\vcpkg.exe add artifact microsoft:pico-sdk
warning: vcpkg-artifacts are experimental and may change at any time.
 Artifact                        Version    Status        Dependency  Summary
 microsoft:compilers/arm/gcc     2020.10.0  installed                 GCC compiler for ARM CPUs.
 microsoft:tools/kitware/cmake   3.20.1     installed                 Kitware's cmake tool
 microsoft:raspberrypi/pico-sdk  1.3.0      will install              The Raspberry Pi Pico SDK

Run `vcpkg activate` to apply to the current terminal
PS C:\Dev\test> type .\vcpkg-configuration.json
{
  "default-registry": {
    "kind": "git",
    "baseline": "e2667a41fc2fc578474e9521d7eb90b769569c83",
    "repository": "https://github.com/microsoft/vcpkg"
  },
  "registries": [
    {
      "kind": "artifact",
      "location": "https://aka.ms/vcpkg-ce-default",
      "name": "microsoft"
    }
  ],
  "requires": {
    "microsoft:compilers/arm/gcc": "undefined 2020.10.0",           <-- !!!!!!!!!!!!!!!!!!!!!!!!!!!!!
    "microsoft:tools/kitware/cmake": "undefined 3.20.1",           <-- !!!!!!!!!!!!!!!!!!!!!!!!!!!!!
    "microsoft:raspberrypi/pico-sdk": "* 1.3.0"
  }
}
PS C:\Dev\test>

After:

PS C:\Dev\test> type .\vcpkg-configuration.json
{
  "default-registry": {
    "kind": "git",
    "baseline": "e2667a41fc2fc578474e9521d7eb90b769569c83",
    "repository": "https://github.com/microsoft/vcpkg"
  },
  "registries": [
    {
      "kind": "artifact",
      "location": "https://aka.ms/vcpkg-ce-default",
      "name": "microsoft"
    }
  ],
  "requires": {
  }
}
PS C:\Dev\test> C:\Dev\vcpkg-tool\out\build\x64-Debug\vcpkg.ps1 add artifact microsoft:pico-sdk
warning: vcpkg-artifacts are experimental and may change at any time.
Using in-development vcpkg-artifacts built at: C:/Dev/vcpkg-tool/ce/ce
 Artifact                        Version    Status        Dependency  Summary
 microsoft:compilers/arm/gcc     2020.10.0  installed     *           GCC compiler for ARM CPUs.
 microsoft:tools/kitware/cmake   3.20.1     installed     *           Kitware's cmake tool
 microsoft:raspberrypi/pico-sdk  1.3.0      will install              The Raspberry Pi Pico SDK

Run `vcpkg activate` to apply to the current terminal
PS C:\Dev\test> type .\vcpkg-configuration.json
{
  "default-registry": {
    "kind": "git",
    "baseline": "e2667a41fc2fc578474e9521d7eb90b769569c83",
    "repository": "https://github.com/microsoft/vcpkg"
  },
  "registries": [
    {
      "kind": "artifact",
      "location": "https://aka.ms/vcpkg-ce-default",
      "name": "microsoft"
    }
  ],
  "requires": {
    "microsoft:raspberrypi/pico-sdk": "* 1.3.0"
  }
}
PS C:\Dev\test>