Stop bundler eagerly loading all specs with exts

[segiddins]

We were setting the wrong extension_dir for git specs stubs

Additionally, the call to self.extension_dir was loading the remote spec, which was avoidable since the stub had an extension dir (and in fact its #gem_build_complete_path does exactly what we want anyway)

Finally, now set the base_dir when loading the remote_spec from a stub specification, since the git source sets the base dir for stubs based on where the spec will be installed to, and we want to preserve that so the base_dir for the loaded spec & the stub are the same

What was the end-user or developer problem that led to this PR?

Bundler was eagerly loading all gemspecs that had extensions in their stubs

What is your fix for the problem, implemented in this PR?

Avoid calling methods that force loading the remote spec inside of #missing_extensions?

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[hanyang-tony]

For universal rubies, the change to def gem_build_complete_path is incorrect. Previously, extension_dir is invoked on the Bundler::StubSpecification, which is a child class of Specification. After the change to stub.gem_build_complete_path, @Stub is actually an instance of Gem::StubSpecification, thus gem_build_complete_path is from Gem::BasicSpecification.

The problem arise when in bundler, we replace the 'universal' part of Gem::Platform with the arch. But we still use the universal name for Gem::Specification, this is defined in 'rubygems_ext.rb'. Now the stub is using extension_dir defined in Gem::BasicSpecification, so the 'universal' part ended up being swapped by arch.

So, we install native extensions to 'extensions/universal-darwin-22/', but finding them in 'extensions/x86_64-darwin-22/'. Leading to native extension gems can never be installed on universal ruby.