When a docsite has only one package name and no DocsiteInfo.overview,
treat it like Javadoc's single-module output: redirect the top-level
index to the package page and omit the site-title breadcrumb segment
from generated pages.
Add src/test/files/SinglePackageTest fixtures to cover multiple package
versions, redirect behavior, breadcrumb behavior, and unchanged site
structure.
Also:
- Shut down Executor used in test.
- Declare expected output fixtures of DocGenerator as test inputs, not
outputs.
- Fix IntelliJ warning by using a Set for the right-hand side of
collection subtraction.
odenix