Use combined reference assembly/targeting pack zip file

[lbussell]

Fixes #1135

This PR is marked as draft because we still need to publish the reference assembly zip to our storage account.

Here's the PowerShell script that I used to generate the new reference assembly zip:

$blobs = @(
    "v4.0.zip",
    "v4.5.2.zip",
    "v4.6.2.zip",
    # v4.7.1 is not used and conflicts with 4.7.2's files
    # "v4.7.1.zip",
    "v4.7.2.zip",
    "v4.8.1.zip",
    "v4.8.zip"
)

foreach ($blobname in $blobs) {
    $url = "https://dotnetbinaries.blob.core.windows.net/referenceassemblies/$blobname"
    $output = "./$blobname"
    Invoke-WebRequest -Uri $url -OutFile $output
}

foreach ($blobname in $blobs) {
    Expand-Archive -path "./$blobname" -DestinationPath "./temp" -Force
}

Compress-Archive -Path "./temp/*" -DestinationPath "./referenceassemblies.v0.zip"

[lbussell]

Closing in favor of #1224