pkl-doc: Support single-package docsite mode (#1592)

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.

pkl-doc/src/test/files/SinglePackageTest/output/com.package1/current/package-data.json

@@ -0,0 +1 @@
+{"ref":{"pkg":"com.package1","pkgUri":null,"version":"4.5.6"},"summary":"Additional version fixture for single-package site tests.","sourceCode":"https://example.com/package1/","sourceCodeUrlScheme":"https://example.com/package1%{path}#L%{line}-L%{endLine}","dependencies":[{"ref":{"pkg":"pkl","pkgUri":null,"version":"0.24.0"}}],"modules":[{"ref":{"pkg":"com.package1","pkgUri":null,"version":"4.5.6","module":"minimal"},"summary":"Additional version module fixture.","moduleClass":{"ref":{"pkg":"com.package1","pkgUri":null,"version":"4.5.6","module":"minimal","type":"ModuleClass"},"superclasses":[{"pkg":"pkl","pkgUri":null,"version":"0.24.0","module":"base","type":"Module"},{"pkg":"pkl","pkgUri":null,"version":"0.24.0","module":"base","type":"Typed"},{"pkg":"pkl","pkgUri":null,"version":"0.24.0","module":"base","type":"Object"},{"pkg":"pkl","pkgUri":null,"version":"0.24.0","module":"base","type":"Any"}]},"classes":[{"ref":{"pkg":"com.package1","pkgUri":null,"version":"4.5.6","module":"minimal","type":"Person"},"superclasses":[{"pkg":"pkl","pkgUri":null,"version":"0.24.0","module":"base","type":"Typed"},{"pkg":"pkl","pkgUri":null,"version":"0.24.0","module":"base","type":"Object"},{"pkg":"pkl","pkgUri":null,"version":"0.24.0","module":"base","type":"Any"}]}]}]}