Fix two issues by including the ruby platform variants in the lock file

[rwstauner]

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

2 issues:

• When the lock file only contains platform specific variants but install is being done by a version of ruby that doesn't work with the platform version bundler will install the ruby variant... but then at setup time it errors because it looks for the platform variant and it isn't there. bundler doesn't need to install an alternative that doesn't satisfy it's own requirements. The first commit adds a test that verifies this behavior (the second commit updates the test to fail earlier but with a suggestion people can use to resolve the issue).
• It's been a long standing issue that you can't easily retain the ruby platform variant of some gems (by adding "ruby" to the list of platforms) if not all gems support it. This separates those two concepts by locking the ruby platform variant without requiring all gems to support the "ruby" platform. By updating your lockfile this way you can resolve the previous issue (and also stop having to edit the lock file by hand).

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

Keep the ruby variants and their dependencies in the lockfile when they would be useful as a fallback regardless of whether the ruby platform is included.

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