Initial project references changes (#11648)

* first changes for showing project references

* add tests

* fix interface and formatting

* add try so that project still gets loaded even if dependency project loading fails

* use instanceof

* add circular reference error

extensions/sql-database-projects/src/common/constants.ts

@@ -118,6 +118,7 @@ export function fileAlreadyExists(filename: string) { return localize('fileAlrea
 export function folderAlreadyExists(filename: string) { return localize('folderAlreadyExists', "A folder with the name '{0}' already exists on disk at this location. Please choose another name.", filename); }
 export function invalidInput(input: string) { return localize('invalidInput', "Invalid input: {0}", input); }
 export function unableToCreatePublishConnection(input: string) { return localize('unableToCreatePublishConnection', "Unable to construct connection: {0}", input); }
+export function circularProjectReference(project1: string, project2: string) { return localize('cicularProjectReference', "Circular reference from project {0} to project {1}", project1, project2); }
 
 export function mssqlNotFound(mssqlConfigDir: string) { return localize('mssqlNotFound', "Could not get mssql extension's install location at {0}", mssqlConfigDir); }
 export function projBuildFailed(errorMessage: string) { return localize('projBuildFailed', "Build failed. Check output pane for more details. {0}", errorMessage); }
@@ -166,6 +167,8 @@ export const DatabaseVariableLiteralValue = 'DatabaseVariableLiteralValue';
 export const DSP = 'DSP';
 export const Properties = 'Properties';
 export const RelativeOuterPath = '..';
+export const ProjectReference = 'ProjectReference';
+export const TargetConnectionString = 'TargetConnectionString';
 
 // SqlProj File targets
 export const NetCoreTargets = '$(NETCoreTargetsPath)\\Microsoft.Data.Tools.Schema.SqlTasks.targets';