move the routines that find the matching scheme path into their own methods.

This `populate` method is getting pretty big - let's refactor it a bit.
move the routines that populate buildable + testables into their own methods.

This populate function was getting too long.
the generated scheme for tests should match the original scheme's settings.

If the original scheme enabled 'parallelizeBuildables' or
'buildImplicitDependencies', then the generated scheme should use them,
too.
the generated Tests workspace should include all projects in subject's workspace

Previously, we only added projects for the buildables and testables to
the generated workspace.  This was fine, but it doesn't give Xcode's
Find Implicit Dependencies feature enough information to work from.
Let's make the generated workspace include all the projects.

Test Plan:
Tests pass.  Verified that find implicit dependencies now works with a
CocoaPods project.
Merge pull request #98 from facebook/fpotter/fix-implicit-dependencies

Make xctool play nice with Find Implicit Dependencies