Use MSBuildProjectExtensionsPath for RestoreOutputPath

[dsplaisted]

Replaces #2056 with less refactoring and simplified commit history.

This is part of a solution to dotnet/msbuild#1603. The MSBuildProjectExtensionsPath is where MSBuild will automatically import project .props and .targets files from. This PR changes the RestoreOutputPath to default to MSBuildProjectExtensionsPath instead of BaseIntermediateOutputPath. By default, both of these properties will default to the obj folder, but this change will make the behavior more correct if one of the properties is overridden.

This PR also sets the EnableBaseIntermediateOutputPathMismatchWarning property when appropriate, which enables an MSBuild warning if MSBuildProjectExtensionsPath doesn't match BaseIntermediateOutputPath (added in dotnet/msbuild#3059).

Included in this PR is also a commit that replaces new string[0] with Array.Empty<string>(), which was something I noticed when browsing the code investigating this change.

@nkolev92

[jainaashish]

looks good, great work!

[nkolev92]

The test results made no sense, so I kicked them off again.

[jainaashish]

As we discussed in the meeting earlier, there are 2 options here:

1. Use current field BaseIntermediatePath to specify MSBuildProjectExtensionsPath value so that won't change anything in interface but change the meaning of this property.

2. Although we dont have any dictionary at IVsProjectRestoreInfo but we do have a dictionary at IVsProjectRestoreInfo -> IVsTargetFrameworks -> IVsTargetFrameworkInfo -> IVsProjectProperties which is to provide project properties per TFM. So Project system can add MSBuildProjectExtensionsPath at this level and duplicate it for each TFM.

Both these options aren't ideal, but if I've to choose one then I'll vote for approach#1 since option# 2 exposes more vulnerability.

@rrelyea @nkolev92

[dsplaisted]

@jainaashish @nkolev92 I've updated the IVsProjectRestoreInfo to remove the additional property I added and redefine the meaning of its BaseIntermediatePath property.

I think you can take it from here

[nkolev92]

Kicked off the tests again.