[xaprepare] Restore for multiple solutions (dotnet#3997)

Context: dotnet#3884

Commit 91467bb updated the build system so that instead of trying to
do "everthing" within a single `.sln` file -- which would result in
various file share exceptions when attempting to build
`Xamarin.Android.sln` within Visual Studio on Windows -- the build
tree would instead become "stateful":

 1. Build `Xamarin.Android.BootstrapTasks.sln`.
 2. Then `Xamarin.Android.sln` can be built.
 3. Then `Xamarin.Android-Tests.sln` can be built.

(1) was handled "internally" via `msbuild /t:Prepare`.

A result of 91467bb is that if e.g.
`build-tools/Xamarin.Android.Tools.BootstrapTasks` were changed, building
`src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj`
would no longer cause `Xamarin.Android.Tools.BootstrapTasks` to be
rebuilt.

(This was the point, as rebuilding
`Xamarin.Android.Tools.BootstrapTasks`/etc. is what caused the file
sharing issues that we wanted fixed.)

Overlooked in 91467bb is that `xaprepare` (invoked by
`msbuild /t:Prepare`) is responsible for restoring NuGet packages on
`.sln` files, but it would only restore packages for
`Xamarin.Android.sln`.

This restriction caused problems in PR dotnet#3884, which attempted to add
NuGet packages to `build-tools/Xamarin.Android.BootstrapTasks`, but
because that solution never had its NuGet packages restored,
attempting to use the expected *outputs* of NuGet restore would fail:

	$ cat build-tools/Xamarin.Android.BootstrapTasks/packages.config 
	<?xml version="1.0" encoding="utf-8"?>
	<packages>
	  <package id="Microsoft.DotNet.ApiCompat" version="5.0.0-beta.19602.1" targetFramework="net472" />
	  <package id="Microsoft.DotNet.GenAPI" version="5.0.0-beta.19602.1" targetFramework="net472" />
	  <package id="Mono.Posix.NETStandard" version="1.0.0" targetFramework="net472" />
	  <package id="Xamarin.LibZipSharp" version="1.0.6" targetFramework="net472" />
	</packages>
	$ msbuild -t:restore build-tools/Xamarin.Android.BootstrapTasks/Xamarin.Android.Tools.BootstrapTask.csproj
	…
	$ ls packages/Microsoft.D*
	ls: cannot access 'packages/Microsoft.D*': No such file or directory

Improve build system sanity: when `xaprepare` restores NuGet packages,
it should restore the following solutions:

  * `Xamarin.Android.BootstrapTasks.sln`
  * `Xamarin.Android.Build.Tasks.sln`
  * `Xamarin.Android.sln`

This will allow PR dotnet#3884 to work as intended.

build-tools/xaprepare/xaprepare/Application/Context.cs

@@ -18,8 +18,15 @@ partial class Context : AppObject
 		public const ConsoleColor FailureColor                = ConsoleColor.Red;
 		public const ConsoleColor WarningColor                = ConsoleColor.Yellow;
 
-		static readonly string XASolutionFilePath             = Path.Combine (BuildPaths.XamarinAndroidSourceRoot, "Xamarin.Android.sln");
-		static readonly string XATestsSolutionFilePath        = Path.Combine (BuildPaths.XamarinAndroidSourceRoot, "Xamarin.Android-Tests.sln");
+		static readonly IEnumerable<string> XASolutionFilesPath         = new string [] {
+			Path.Combine (BuildPaths.XamarinAndroidSourceRoot, "Xamarin.Android.BootstrapTasks.sln"),
+			Path.Combine (BuildPaths.XamarinAndroidSourceRoot, "Xamarin.Android.Build.Tasks.sln"),
+			Path.Combine (BuildPaths.XamarinAndroidSourceRoot, "Xamarin.Android.sln"),
+		};
+
+		static readonly IEnumerable<string> XATestsSolutionFilesPath    = new string [] {
+			Path.Combine (BuildPaths.XamarinAndroidSourceRoot, "Xamarin.Android-Tests.sln"),
+		};
 
 		string logDirectory;
 		string mainLogFilePath;
@@ -171,12 +178,12 @@ partial class Context : AppObject
 		/// <summary>
 		///   Path to the Xamarin.Android solution file
 		/// </summary>
-		public string XASolutionFile                      => XASolutionFilePath;
+		public IEnumerable<string> XASolutionFiles        => XASolutionFilesPath;
 
 		/// <summary>
 		///   Path to the Xamarin.Android tests solution file
 		/// </summary>
-		public string XATestsSolutionFile                 => XATestsSolutionFilePath;
+		public IEnumerable<string> XATestsSolutionFiles   => XATestsSolutionFilesPath;
 
 		/// <summary>
 		///   If <c>true</c>, the current console is capable of displayig UTF-8 characters

build-tools/xaprepare/xaprepare/Steps/Step_PrepareExternal.cs

@@ -14,13 +14,18 @@ protected override async Task<bool> Execute (Context context)
 		{
 			var nuget = new NuGetRunner (context);
 
-			if (!await NuGetRestore (nuget, context.XASolutionFile)) {
-				return false;
+			foreach (var solutionFile in context.XASolutionFiles) {
+				if (!await NuGetRestore (nuget, solutionFile)) {
+					return false;
+				}
 			}
 
 			Log.StatusLine ();
-			if (!await NuGetRestore (nuget, context.XATestsSolutionFile)) {
-				return false;
+
+			foreach (var solutionFile in context.XATestsSolutionFiles) {
+				if (!await NuGetRestore (nuget, solutionFile)) {
+					return false;
+				}
 			}
 
 			var msbuild = new MSBuildRunner (context);