Memory usage improvements in build (#2626)

* Use a WorkspaceContext in dotnet-build to cache project data across
multiple compilations in a single build action
* Dramatically reduce string and object duplication by introducing a
"Symbol Table" that shares instances of NuGetVersion, NuGetFramework,
VersionRange and string across multiple lock-file parses

Test Results:
* Testing was done by compiling Microsoft.AspNetCore.Mvc (and it's
dependencies) and taking memory snapshots after each compilation in
dotMemory
* We used to allocate ~3MB and deallocate ~2.5MB on EACH compilation in
a single build action. This has been reduced to ~120KB
allocated/deallocated
* After introducing WorkspaceContext, total memory usage spiked from 6MB
across the whole build action to about 13MB, introducing the symbol
table dropped it back to about 5-6MB.

scripts/dotnet-cli-build/CompileTargets.cs

@@ -280,7 +280,7 @@ namespace Microsoft.DotNet.Cli.Build
             return rid;
         }
 
-        [Target]
+        [Target(nameof(PrepareTargets.Init))]
         public static BuildTargetResult CompileStage1(BuildTargetContext c)
         {
             CleanBinObj(c, Path.Combine(c.BuildContext.BuildDirectory, "src"));
@@ -305,7 +305,7 @@ namespace Microsoft.DotNet.Cli.Build
             return result;
         }
 
-        [Target]
+        [Target(nameof(PrepareTargets.Init))]
         public static BuildTargetResult CompileStage2(BuildTargetContext c)
         {
             var configuration = c.BuildContext.Get<string>("Configuration");