feat: add a cwd build option

[dsherret]

Adds a cwd build option so that dnt can operate on a directory other than the process's current working directory:

await build({
  cwd: "./packages/add",
  entryPoints: ["./mod.ts"], // ./packages/add/mod.ts
  outDir: "./npm",           // ./packages/add/npm
  // ...etc...
});

It defaults to Deno.cwd(), so existing builds are unaffected.

There were three separate places relying on the process cwd:

1. standardizePath resolved entryPoints, outDir, and testPreloadModule against it.
2. The test file glob used rootTestDir ?? Deno.cwd() as its root (this is what causes an unrelated subdirectory of the cwd to get pulled into a build).
3. It was passed to the transform as the workspace root, which is what discovers a deno.json, package.json, and deno.lock when the entry points are all remote.

All three now resolve from the cwd option. Along the way:

• The lower level transform() API now resolves its own relative paths (entry points, mappings, importMap, configFile, module shims) from its existing cwd option instead of the process cwd, and accepts a plain path there in addition to a file url.
• rootTestDir was previously handed to expandGlob unresolved, so a relative value resolved against the process cwd. It now resolves from the cwd option.
• mapMappingKey's existsSync(key) fallback was probing the process cwd.

Closes #337