When importing file entities, check for duplicate files

Problem/Motivation

In #3292287: Extend recipe runner to create content provided in the recipe's /content folder, we're adding a way to import content provided by a recipe. This is based on parts of the contributed Default Content module.

In review, @alexpott pointed out that, when importing file entities, the associated file is copied into the site's files directory. If there are duplicates, you end up with many copies of the same file. That sorta sucks.

Proposed resolution

Detect if we already have a particular file in the files directory, and don't import it if we do.