Fix AWS patcher loading wrong constants when AWS is partially loaded

[delner]

Resolves #938.

There's a scenario when AWS is partially loaded such that certain constants are not available, const_get will search ancestors and use any matching constant there, which results in the wrong constant being patched.

Additionally, despite providing a false flag to disable ancestor search, Module#const_get seems to ignore this sometimes when Rails autoloading is present (at least the first time), causing it to still unexpectedly return the wrong constant.

e.g.

module Support
  class Client
  end
end

module Aws
  # Not loaded
  # module Support
  #   class Client
  #   end
  # end
end

::Aws.const_get(:Support).const_get(:Client)
# => Support::Client
::Aws.const_get(:Support, false).const_get(:Client, false)
# In Ruby env, => NameError
# In Rails env, can result in Support::Client on first invocation.

In this PR we make sure to cross check the loaded constants against the list, then only collect what's actually available, rather than depending on const_get to resolve any namespace conflicts.

[delner]

@illdelph you can test this with the following pre-release gem:

source 'http://gems.datadoghq.com/prerelease' do
  gem 'ddtrace', '0.32.0.fix.aws.constant.loading.55008'
end

[illdelph]

source 'http://gems.datadoghq.com/prerelease' do
gem 'ddtrace', '0.32.0.fix.aws.constant.loading.55008'
end

Yeah, that seems to do the trick