This provides better debugger strings for the RazorProjectItems that represent default imports.

Some were sealed, some weren't.

This change makes the IImportProjectFeature interface internal. In addition, the SetImportFeature extension method on RazorProjectEngineBuilder has been moved to the common test library, since it's really intended to be used by tests.

This is a public API breaking change, but there are no users of these APIs in dotnet/sdk.

Consolidate the three "TestImportProjectFeature" classes and place it in Microsoft.AspNetCore.Razor.Test.Common.

This change introduces a DefaultImportProjectItem that is shared across features that add default imports.

Add a couple of methods to PooledArrayBuilder to produce a reversed ImmutableArray.

This change rewrites RazorProjectFileSystem.FindHierarchicalItems(...) to avoid allocations. It now returns results in reverse order, since every caller in Razor immediately called `Reverse()`. In addition, the access has been reduced to internal since there are no external callers of this method.

FindHierarchicalItem has a "max depth" check to ensure that it returns a maximum of 255 file paths. However, all this method does is perform string manipulation by looking for '/' characters. It does not actually touch the file system.

Technically, this could result in a breaking change because this method is used to find applicable Imports files. So, if the user had a Razor project with 256+ nested subdirectories, the compiler would now consider Imports that it might not have considered in the past. However, that seems like a highly unlikely situation and it's even more unlikely that a user would depend on that behavior.

This change represents a lot of clean up and consolidate a lot of code
around how imports are provided and queried for in the compiler.

There are a couple of API breaking changes here that shouldn't have
external impact. In particular, I've made `IImportProjectFeature`
internal and changed its API significantly. In addition, I've rewritten
`RazorProjectFileSystem.FindHierarchicalItems(...)` to reduce
allocations, make it more efficient, and return its results in reversed
order (since every caller immediately called `Reverse()`).

CI Build:
https://dev.azure.com/dnceng/internal/_build/results?buildId=2626377&view=results
Test Insertion:
https://dev.azure.com/devdiv/DevDiv/_git/VS/pullrequest/605137