Implement `std::Error::source()` for errors

[honzasp]

ModuleGraphError and ResolutionError did not implement Error::source(), so if a user-provided Resolver or Loader returns an error, its cause chain is lost.

This PR implements source(), but it keeps the existing implementation of Display. Unfortunately, this means that when anyhow prints the cause chain of errors, it might print some error messages multiple times. For example, when we print a ModuleGraphError::LoadingErr(err), then anyhow first prints the ModuleGraphError, which displays err. anyhow then prints the source of the ModuleGraphError, which is err, so err is displayed again!

There are two possible solutions to this problem:

1. Change implementation of Display, so that ModuleGraphError::LoadingErr(err) displays a fixed error message ("The module could not be loaded"). The disadvantage is that if the caller does not also print the cause chain, the user will not see the actual error!
2. Change implementation of Error::source(), so that ModuleGraphError::LoadingErr(err) returns err.source() instead of err. This means that err will not be displayed twice, but err will no longer be present in the cause chain.

[CLAassistant]

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