Modify default gemspec registration to support unmodified specs.

MRI 2.0 shipped with its own default spec generator that rewrites
the gemspec file to contain a list of bare require names in
spec.files. This required extra logic in rbinstall.rb and makes
it impossible to simply copy a gemspec from a gem into the default
directory to register that gem as a default gem.

PR #566 adds --default functionality to `gem install` that copies
the gem's unmodified gemspec to the default directory, but since
unmodified gemspec contains a list of gem-relative filenames, it
could not use the old default specification logic.

This commit modifies default spec registration to support both
formats. In the "old" style, specifications are registered with
their raw filenames; we detect old style by checking the first
file to see if it is prefixed with any spec.require_paths. In the
"new" style, spec.require_paths are stripped off the spec's
filenames before registration, and only files that start with a
require_path are registered.

Both the old format and the new format continue to work with this
change, and we can migrate toward default specs being the
unmodified originals rather than custom-built as in MRI 2.0.