Refactor SDK targets to address multiple writes and flaky builds

[jviau]

Issue describing the changes in this PR

resolves #1834

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• I have added all required tests (Unit tests, E2E tests)

Additional information

This PR is a near complete overhaul of our SDK targets. The aim of the changes is to integrate more closely with existing dotnet SDK targets to eliminate redundant work.

1. Most Copy tasks have been removed and instead we augment specific item groups so that CopyToOutputDirectory and CopyToPublishDirectory tasks will copy our files for us.
2. Incremental build support has been improved, we now use Inputs and Outputs.

• Some incremental build optimizations are thwarted by certain targets from the extension inner build. For example, extensions.json is always written to, so we always see it as changed and also write to it.

3. Targets have been shifted to perform all incremental steps into the IncrementalOutputPath. Only copying the final files in one go as part of the existing CopyToDirectoryDirectory (or publish dir).
4. For publish, we never actually call publish on the inner WorkerExtensions.csproj. Instead, we can rely on the existing build targets and collecting of files to accomplish the same thing more efficiently.

Concerns

This is a breaking change for anyone that has customized our targets. Hence the draft state. I am going to go through this and see if I can rename targets back to existing ones to minimize or eliminate the breaking change. See comment below

[jviau]

Actually, after reviewing the changes technically this is not breaking. The convention in MSBuild is any property, target, or item starting with _ is considered private. All of our targets start with _ so nothing public is changing.

[brettsam]

Reviewing these msbuild targets is tough! I think my only comment is that I'd like to see some comments sprinkled throughout the new Targets explaining what they're doing so it's easier for other folks to find the right place to update in the future.