init. Removed dependency on MSBuildLocator.RegisterDefaults().

[deepchoudhery]

We were using Microsoft.Build.Locator to load in requisite Microsoft.Build assemblies (using MSBuildLocator.RegisterDefaults()) to :

• initialize Microsoft.Build.Evaluation.Project object
• use Project.GetProperty("ms build property name")?.EvaluatedValue to get needed MSBuild project properties
• we needed the "Nullable" property.

Now:

• Added Nullable string property to IProjectContext
• Initialize IProjectContext during runtime msbuild operation using updated VS.Web.CG.Msbuild/Target/build/Microsoft.VisualStudio.Web.CodeGeneration.Tools.targets file.
• Nullable property populated at runtime without performing additional Microsoft.Build initialization and calls.

Changes :

• Removed package references to Microsoft.Build.Locator
• Removed all calls to MSBuildLocator.RegisterDefaults().
• Added tests with nullable = enable, disable and the nullable property missing (disable by default).

[zahalzel]

LGTM