Make sure the output package cleanup happens before Pack

Because of the way we were constructing the `_PrivateAssets` and `_ShouldPack` properties, we were ending with duplicates (i.e. `all;all`) if the package reference was both direct and indirectly included, since the previous item transform would basically concatenate all such values, breaking the condition in the subsequent item group.

Rather than using an item transformation, we now instead just reference the item metadata directly, which will result in MSBuild iterating all items and assigning the property multiple times, once for each item. The last such item will determine the final property value. Therefore, we invert the Implicit references so they are processed first, so that an explicit `PackageReference.PrivateAssets` or `Pack` metadata trumps implicitly defined reference values.

A new test ensures this is the case (it would fail without the fix) with the Shell.Interop VSSDK package.

Fixes #28

First and most important: add unicode icons!
Second: instead of hardcoding `preview` as the VS being searched, leverage the ability to select the first instance greater than or equal to 16.8 we find on the machine. This means we won't be force-installing a preview VS when 16.8 becomes the built-in version in hosted agents. Also, it means when 16.9 or future versions become stable too, we won't need to modify scripts again, since we'll automatically use a later version this way.

The default (powershell) for Windows doesn't seem to be working for adding envvar/path

The item metadata on a `PackageReference` may be useful to consumers that use `PrivateAssets` to automatically add implicit package references to transitive dependencies (i.e. to include all dependent assemblies as part of a build tasks assembli). The user-defined metadata may later be used to further tweak the inference (i.e. selectively removing some items from the resulting `PackageFile` items). For this scenario, it's useful to preserve the original metadata in the top-most `PackageReference` that triggers an implicit transitive dependency tree to be a candidate for inference.

We also remove from the task the conditional checks on the package reference metadata since the targets already performed the same checks. This makes the task more flexible, should a user decide to invoke it explicitly as part of some custom inference they might want to perform (i.e. inferring content based on different metadata than `PrivateAssets=all`).

Add support for a `PackageReference` metadata attribute on `PackageFile` so that content from other packages can be included more easily.

Fixes #30.

This file is only used for nugetizer tasks build itself.

Due to the way we were setting up the BuildDependsOn when PackOnBuild == true, we were running Pack after build, regardless of whether the build was single or multi-targeting. This caused the Pack to run once for each TF, causing each build to overwrite the previous package and therefore leaving only the last TFM built as the package contents.

This change switches to using Before/After targets and conditions instead with DependsOn for Build/Pack, as well as a mirror property to PackOnBuild, BuildOnPack to avoid building before pack (which the default behavior in SDK pack too).

Fixes #32.

Dev > Main

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