[Build] Packing `SourceGen` projects fail when using a custom `OutputPath`

## Problem

When building and packing the MVVM `SourceGenerator` projects, pack fails to find the output artifacts when we use custom output paths. This is observed in #96 when I tried to use a common output path for building the entire solution. I could update the current method to find the artifacts **but it feels hacky and not integrated with the build pipeline**.

Thus, any change like changing the output paths not just in the build definition but also changing via terminal invocation (_say `dotnet build -o path_to_out_dir`_) will definitely break the build and it won't pack those files. **Worst of all, sometimes, the build pipeline won't fail.** So, we won't know if the MVVM package was properly packed.

## To Reproduce

1. Clone the repo or clean it
`git clean -Xdf`
2. Build MVVM SourceGenerator project (_with a different output path_).
`dotnet build CommunityToolkit.Mvvm.SourceGenerators -o bin_dir`
3. Pack the MVVM project (_with a different output path_).
`dotnet pack CommunityToolkit.Mvvm -o pkg_dir`

## Result

| Expected | Actual |
| --- | --- |
| Build and pack should have succeeded and the outputs of the MVVM SourceGenerator project should be in the target path of NuGet package generated from the MVVM parent project. | Pack in the final step fails (_in a clean repo but succeeds when older artifacts are present_). When it does succeed, we either find the older artifacts or no artifacts in the NuGet package of the parent MVVM project. |

## Solution

Get the build outputs dynamically via `GetBuildOutputs` MSBuild target mirroring NuGet's Pack targets and add it to pack via `_PackageFiles ` internal MSBuild item used by NuGet's Pack targets. This is brittle only if @NuGet team decides to modify/remove `_PackageFiles` which is highly unlikely.

One interesting side-effect of this implementation is that when we decide to multi-target, we can easily modify our solution to be more resilient without having to tackle the problem again since we already output the artifacts by TFM. ___This should save hours of headbutting against the wall___ 😉!!