date "last Modified" doesn't take in consideratoin includes

[jalberto]

Describe the bug
when using date: Last Modified in a liquid file that includes other files, the date doesn't change when only an include is being modified

To Reproduce

• create a file with 3 includes
• modify any of the includes but not the parent file
• you get the old date

Expected behavior
Last-Modified should take into consideration compiled result for date

Environment:

• Linux
• Eleventy Version 0.10.0

[uncenter]

This would require Eleventy to analyze what templates are included/rendered (dependencies) in order to use those date(s) as the last modified date. Related to harttle/liquidjs#233, though that solution (rendering the template with a fake filesystem to capture read calls) does not seem viable.