[rubygems/rubygems] Avoid duplicated generation of APISpecification objects

As far as I could see, `Gem::Resolver::APISpecification` objects are
supposed to be immutable.  If my guessing is correct, then we can cache
and reuse its instances for performance.
At least, `rake` passes on my machine.

Before this change:

```
$ time ruby -I lib bin/gem install --no-doc aws-sdk
Successfully installed aws-sdk-3.0.1
1 gem installed
real    0m37.104s
user    0m36.952s
sys     0m0.333s
```

After this change:

```
$ time ruby -I lib bin/gem install --no-doc aws-sdk
Successfully installed aws-sdk-3.0.1
1 gem installed
real    0m23.905s
user    0m23.740s
sys     0m0.365s
```

https://github.com/rubygems/rubygems/commit/7e8fbba85c

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