Item batching can be very slow, especially on Linux

Changing an item include to include some additional metadata resulted in extreme slowdowns for me, especially on Linux:

OS | Without metadata | With metadata
---- | - | -
Windows | 10 ms | 8234 ms
Linux | 68 ms | 143022 ms

The change that caused this was changing from:

```xml
<_RuntimeTargetPublishItems Include="@(FileDefinitions)" Exclude="@(__RuntimeTargetPublishItems)" />
```

To:

```xml
<_RuntimeTargetPublishItems Include="@(FileDefinitions)" Exclude="@(__RuntimeTargetPublishItems)"
DestinationSubPath="%(FileDefinitions.Path)" />
```

# Repro steps

- `git clone https://github.com/dsplaisted/msbuild-slow-batching`
- `cd msbuild-slow-batching`
- `cd TestApp`
- `dotnet build /clp:PerformanceSummary`
- Observe the difference in execution time between the FastBatching and SlowBatching targets in the performance summary

EXPECTED: Execution times are comparable
ACTUAL: SlowBatching execution time is several orders of magnitude slower than FastBatching

# Repro details

The repro project is an app targeting `netcoreapp1.1` referencing a library which targets `netstandard1.5`. The app project has the following targets added to it:

```xml

```