Factor out common parts of types/{Library,ForeignLibrary,Executable,Benchmark,TestSuite}.dhall

[quasicomputational]

Most of all of those files are what we might call 'BuildInfo', following Cabal, plus a few component-specific fields. When that code was written, Dhall didn't have a way to combine record types. But now the //\\ operator is a thing, and we can factor out all 28 or so shared fields (plus another 10 when #42 lands) into a new file types/BuildInfo.dhall, and then define them with reference to that plus their idiosyncratic fields.

This has an additional benefit because then you can define libraryBuildInfo : Library -> BuildInfo, executableBuildInfo, etc, and then write mapBuildInfo : (BuildInfo ->Config -> BuildInfo) -> Package -> Package to, e.g., conveniently add warnings to all components.

It'd be nice if --print-type would output the build components in terms of BuildInfo too.