Refactor action execution for reuse

Introducing new actions for the GitHub dependency graph will involve reuse of much of
the action infrastructure. This commit reorganises things a little to facilitate reuse.

src/repository-layout.ts

@@ -0,0 +1,16 @@
+import * as core from '@actions/core'
+import * as path from 'path'
+
+export function workspaceDirectory(): string {
+    return process.env[`GITHUB_WORKSPACE`] || ''
+}
+
+export function buildRootDirectory(): string {
+    const baseDirectory = workspaceDirectory()
+    const buildRootDirectoryInput = core.getInput('build-root-directory')
+    const resolvedBuildRootDirectory =
+        buildRootDirectoryInput === ''
+            ? path.resolve(baseDirectory)
+            : path.resolve(baseDirectory, buildRootDirectoryInput)
+    return resolvedBuildRootDirectory
+}