`executables` are often extracted from the `files` in gemspec files.

Use the actual files unpacked from the gem.  The recent rdoc.gemspec
uses different code than expected by rbinstall.rb, which resulted in
the result list not being overwritten and the template files not being
installed.

Since RubyGems 3.0.0 `dir_mode` option is supported, but using `File`
method to apply it, not `FileUtils`.  To reduce overwriting existing
methods (especially built-in class), and to record making directories,
keep using `no_write` method.

The glob patterns are replaced by rote, but these files have been
removed before it.

* Simplified to find gemspecs for bundled gems

Co-authored-by: Nobuyoshi Nakada <nobu.nakada@gmail.com>