Allow users to supply a custom SDK and source-built artifacts

This change tries to implement something analogous to
dotnet/source-build#1455 for
source-build-reference-packages.

With this change, I can build source-build-reference-packages completely
offline on linux-x64 against a source-built SDK by running:

    ./build.sh --with-sdk /usr/lib64/dotnet \
               --with-packages /usr/lib64/dotnet/source-built-artifacts/*.tar.gz

build.sh

@@ -1,8 +1,89 @@
 #!/usr/bin/env bash
 
+set -euo pipefail
+IFS=$'\n\t'
+
+usage() {
+    echo "usage: $0 [options]"
+    echo "options:"
+    echo "  --with-packages <dir>              use the specified archive of source-built packages to build."
+    echo "  --with-sdk <dir>                   use the specified SDK to build."
+}
+
+function abspath() {
+  # $1 : relative filename
+  parentdir=$(dirname "$1")
+
+  if [ -d "$1" ]; then
+      echo "$(cd "$1" && pwd)"
+  elif [ -d "${parentdir}" ]; then
+    echo "$(cd "${parentdir}" && pwd)/$(basename "$1")"
+  fi
+}
+
 __scriptpath=$(cd "$(dirname "$0")"; pwd -P)
 __log_path="$__scriptpath/artifacts/log"
 
+CUSTOM_SDK_DIR=''
+CUSTOM_SOURCE_BUILT_PACKAGES=''
+
+while :; do
+    if [ $# -le 0 ]; then
+        break
+    fi
+
+    lowerI="$(echo $1 | awk '{print tolower($0)}')"
+    case $lowerI in
+        --with-sdk)
+            CUSTOM_SDK_DIR="$2"
+            if [ ! -d "$CUSTOM_SDK_DIR" ]; then
+                echo "Custom SDK directory '$CUSTOM_SDK_DIR' does not exist"
+            fi
+            if [ ! -x "$CUSTOM_SDK_DIR/dotnet" ]; then
+                echo "Custom SDK '$CUSTOM_SDK_DIR/dotnet' not found or not executable"
+            fi
+            shift
+            ;;
+        --with-packages)
+            CUSTOM_SOURCE_BUILT_PACKAGES="$2"
+            if [ ! -f "$CUSTOM_SOURCE_BUILT_PACKAGES" ]; then
+                echo "Custom source-built packages file '$CUSTOM_SOURCE_BUILT_PACKAGES' does not exist"
+                exit 1
+            fi
+            CUSTOM_SOURCE_BUILT_PACKAGES="$(abspath "$CUSTOM_SOURCE_BUILT_PACKAGES")"
+            shift
+            ;;
+        -?|-h|--help)
+            usage
+            exit 0
+            ;;
+        *)
+            echo "Unrecognized argument '$1'"
+            usage
+            exit 1
+            ;;
+    esac
+
+    shift
+done
+
+if [ -n "${CUSTOM_SDK_DIR}" ]; then
+    rm -rf $__scriptpath/.dotnet
+    # TODO we could use ln -s to save space/time, if we never need to write to this directory
+    cp -a "${CUSTOM_SDK_DIR}" $__scriptpath/.dotnet
+    sdk_version="$(basename "$(find "${CUSTOM_SDK_DIR}/sdk" -maxdepth 1 -type d -printf "%p\n" | sort -rn | head -1)")"
+    echo "Found SDK version $sdk_version in ${CUSTOM_SDK_DIR}"
+    jq ".tools.dotnet = \"$sdk_version\"" global.json > global.json.tmp && mv global.json.tmp global.json
+fi
+
+
+if [ -n "${CUSTOM_SOURCE_BUILT_PACKAGES}" ]; then
+    mkdir -p source-built
+    pushd source-built
+    tar xf "${CUSTOM_SOURCE_BUILT_PACKAGES}"
+    popd
+fi
+
 # Use Arcade script to initialize dotnet cli only
 source ./eng/common/tools.sh
 InitializeDotNetCli true