File types should work with EnC

[RikkiGibson]

Related to #60819

• SymbolKey needs to be updated to encode and resolve file types from source and metadata.
• EnC needs to be able to relate file types from one generation to the next, even if the order of files changes. (No need to be resilient to the file name changing--that can be treated as a delete+insert rather than a modification.)

[tmat]

EnC needs to be able to relate file types from one generation to the next, even if the order of files changes

I don't think that's the best approach here. It'd be much simpler if the compiler did not rely on order of syntax trees in the compilation.

[RikkiGibson]

We addressed this in 17.4, but further issues were observed which we think we can address at a later date. Tracked in #64098.