Consider usage files in the GHCi recompilation check

Since a seemingly-growing amount of Haskell code is using Template Haskell to read and include data from external sources, it would be nice if GHC recognized a pragma for specifying additional build dependencies for a source file. Such a pragma could be required to appear before the module headers, and look like:

{-# DEPENDS "file.dat" #-}

ghc --make would add it to other dependencies when deciding what modules to rebuild. The file would NOT be one that's generated by some other build step, but rather play the role of an additional source file, so it would have no effect on the order of compiling modules. My gut says to just make it an error if the file does not exist.

One example of a place this is called for is Michael Snoyman's Yesod and external hamlet files (e.g., hamletFile in the hamlet package). These are plastered all over the place with warnings that if you use them, GHC will no longer automatically rebuild everything it needs to.

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Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Build System
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