Fix class/module alias document having wrong name

[tompng]

Fixes #1014

Some aliased class/module document had wrong title.
Example: https://docs.ruby-lang.org/en/master/Net/HTTP.html
Title should be class Net::HTTP, Not class Net::HTTPSession.

# Source
module Net
  class HTTP
    ...
  end
  HTTPSession = HTTP
  deprecate_constant :HTTPSession
end

RDoc first generates Net/HTTP.html with the correct content, and then overwrite the same file with aliased-name content.

Fixes:

• Stop aliased class/module document file overwrite original one.
• Fix aliasing-to-toplevel check (because test failed with the above change).

Documents of ruby/ruby that will change:

DidYouMean/Formatter.html
  Title change from DidYouMean::VerboseFormatter
Net/HTTP.html
  Title change from Net::HTTPSession
Net/HTTPFound.html
  Title change from Net::HTTPMovedTemporarily
Net/HTTPPayloadTooLarge.html
  Title change from Net::HTTPRequestEntityTooLarge
Net/HTTPRangeNotSatisfiable.html
  Title change from Net::HTTPRequestedRangeNotSatisfiable
Net/HTTPRequestURITooLong.html
  Title was Net::HTTPRequestURITooLarge, file deleted
OpenSSL/PKey/PKeyError.html
  Title change from OpenSSL::PKey::RSAError
Prism/Translation/Parser33.html
  Title change from Prism::Translation::ParserCurrent

[matzbot]

🚀 Preview deployment available at: https://fdc86876.rdoc-6cd.pages.dev (commit: dc4d1a1)

[st0012]

Can you add one of these tests to Aliki’s generator test as well?

[st0012]

Can we have a ClassModule/CodeObject level test to prevent regression too?