Fix included files

[jpenilla]

Fixes included files and adds a test with an imported macro.

The MultiDirectoryLoader treats the source/included directories as if they were merged for resolving macros.

Questions:

• Do we want to have an error when there are multiple files with the same relative path?
• Is this todo in GenerateWorkerInvokerImpl resolved by these changes

    // By default, resolves FS paths
    // todo: restrict inputs to inputs and includes

• Do we want to automatically search for ${templateName}.peb if templateName can't be found in a directory? The Pebble docs show imports being used without a .peb extension.

[zml2008]

looks nice, thanks! the .peb thing looks like something we should do if you've got some time

[jpenilla]

Updated to make the .peb extension in imports optional