Avoid hard-wired build-tree paths

Instead, search for stuff up the directory tree, with the main
functionality being to look for `Gulpfile.js` and assume the resulting
directory is the root.

(Unfortunatley, this is implemented twice, one in `scripts` and another
in `src`.  It's not possible to use a single implementation for both
since that would require assuming a directory structure which this is
intended to avoid.)

Also, in `scripts/build/projects.js`, abstracdt common exec
functionality into a local helper, and use full paths based on the above
search instead of assuming relative paths assuming CWD being in the
project root.

src/harness/findUpDir.ts

@@ -0,0 +1,21 @@
+namespace findUpDir {
+    import { join, resolve, dirname } from "path";
+    import { existsSync } from "fs";
+
+    // search directories upward to avoid hard-wired paths based on the
+    // build tree (same as scripts/build/findUpDir.js)
+
+    export function findUpFile(name: string) {
+        let dir = __dirname;
+        while (true) {
+            const fullPath = join(dir, name);
+            if (existsSync(fullPath)) return fullPath;
+            const up = resolve(dir, "..");
+            if (up === dir) return name; // it'll fail anyway
+            dir = up;
+        }
+    }
+
+    export const findUpRoot: { (): string; cached?: string; } = () =>
+        findUpRoot.cached ||= dirname(findUpFile("Gulpfile.js"));
+}