Investigate including dotnet/icu in source-build 5.0

[dagood]

https://github.com/dotnet/icu showed up as a dotnet/runtime dependency between 5.0 preview 7 and 5.0 preview 8:
https://github.com/dotnet/runtime/blob/v5.0.0-preview.8.20407.11/eng/Version.Details.xml#L7-L10
dotnet/runtime@8c1792d

Based on that, it looks like a browser-wasm dependency. It might not be necessary for 5.0--we don't think we can build our own browser-wasm runtime pack yet (#1721), and runtime packs in general can't even be distributed in the SDK (#1215), so everyone will use the Microsoft-built one from nuget.org for Blazor.

If dotnet/icu bits somehow end up inside the SDK, though, rather than just the runtime pack, we will need to do this to remove prebuilts.

[dseefeld]

Doesn't appear that this is needed for source-build 5.0, since we're not building browser-wasm runtime pack. If prebuilts happen to appear for this because of other dependencies, it can be addressed then.