Proposal for unifying .NET Docker project namespaces

## Summary

We are approaching the point where it would make sense to share code between some of the .NET Docker projects. For example, sharing code to create pull requests between FilePusher and ImageBuilder, or sharing code for reading the manifest.versions.json file between the update-dependencies tool and dotnet-docker tests. Straightening out our namespaces between projects is a prerequisite to that. Even if we didn't decide to do those things, our repos and projects are inconsistent with each other. This is a proposal to make everything more consistent.

## Current

### Repo: dotnet-docker

- Solution: `eng/update-dependencies/update-dependencies.sln`
- Project: `update-dependencies.csproj`
- Namespace: `Dotnet.Docker` [sic]
- Solution: `tests/Microsoft.DotNet.Docker.Tests/Microsoft.DotNet.Docker.Tests.sln`
- Project: `Microsoft.DotNet.Docker.Tests.csproj`
- Namespace: `Microsoft.DotNet.Docker.Tests`

### Repo: docker-tools

- Solution: `eng/src/tools.sln`
- Project: `file-pusher/file-pusher.csproj`
- Namespace: `FilePusher`
- Project: `yaml-updater/yaml-updater.csproj`
- Namespace: `YamlUpdater`
- Solution: `src/Microsoft.DotNet.ImageBuilder/Microsoft.DotNet.ImageBuilder.sln`
- Project: `src/Microsoft.DotNet.ImageBuilder.csproj`
- Namespace: `Microsoft.DotNet.ImageBuilder`
- Project: `tests/Microsoft.DotNet.ImageBuilder.Tests.csproj`
- Namespace: `Microsoft.DotNet.ImageBuilder.Tests`

## Proposal

One solution per repo for our projects (samples notwithstanding). Namespaces should roughly match up with project paths. Give update-dependencies, file-pusher, and yaml-updater more consistent project names and namespaces.

### Repo: dotnet-docker

- Solution: `Microsoft.DotNet.Docker.sln`
- Project: `eng/update-dependencies/Microsoft.DotNet.Docker.UpdateDependencies.csproj`
- Namespace: `Microsoft.DotNet.Docker.UpdateDependencies`
- Project: `eng/Microsoft.DotNet.Docker.Tests.csproj`
- Namespace: `Microsoft.DotNet.Docker.Tests`

### Repo: docker-tools

- Solution: `Microsoft.DotNet.Docker.Tools.sln`
- Project: `src/Microsoft.DotNet.Docker.Tools/FilePusher/FilePusher.csproj`
- Namespace: `Microsoft.DotNet.Docker.Tools.FilePusher`
- Project: `src/Microsoft.DotNet.Docker.Tools/YamlUpdater/YamlUpdater.csproj`
- Namespace: `Microsoft.DotNet.Docker.Tools.YamlUpdater`
- Project: `src/Microsoft.DotNet.Docker.Tools/ImageBuilder/ImageBuilder.csproj`
- Namespace: `Microsoft.DotNet.Docker.Tools.ImageBuilder`
- Project: `src/Microsoft.DotNet.Docker.Tools/ImageBuilder.Tests/ImageBuilder.Tests.csproj`
- Namespace: `Microsoft.DotNet.Docker.Tools.ImageBuilder.Tests`