Clean up build scripts

Decompose into self-contained granular components
Provide reasonable defaults for cross cutting concerns, allowing for independent execution of steps
Start unifying Windows/Bash architecture

fix Bash CI scripts

dockerbuild.sh _common.sh path

Add missing restore-packages.sh

Copy/paste issues

Quote $SOURCE
fix .gitignore

PR Feedback

Merge in @SridarMS's work to avoid redownloading DNX

enabling build of dotnet-build

merge in @SridharMS's CentOS changes

Enable building FSC

enable restoring specific subdirectories

Fix dnx version check

Add missed dependency

Fix pathing to tests

Match Linux build version to Windows, fixing linux tests as a side effect.

workaround for coreclr#2215

fix pathing issue

disable building in docker

BUILD_IN_DOCKER was set, somehow...

fix headers

scripts/test/validate-dependencies.ps1

@@ -0,0 +1,15 @@
+#
+# Copyright (c) .NET Foundation and contributors. All rights reserved.
+# Licensed under the MIT license. See LICENSE file in the project root for full license information.
+#
+
+. "$PSScriptRoot\..\common\_common.ps1"
+
+# Run Validation for Project.json dependencies
+dotnet publish $RepoRoot\tools\MultiProjectValidator -o $Stage2Dir\..\tools
+& "$Stage2Dir\..\tools\pjvalidate" "$RepoRoot\src"
+# TODO For release builds, this should be uncommented and fail.
+# if (!$?) {
+#     Write-Host "Project Validation Failed"
+#     Exit 1
+# }