Refactor package state, also fixing a module reexport bug.
Instead of building a multiply indirected data structure and querying it on every import, we now have two data structures moduleToPkgConf and moduleToPkgConfAll. moduleToPkgConf is a single-level UniqFM that is intended to be used for most valid imports; however, it does not contain any information useful for error reporting. If an error is occurred, we then query moduleToPkgConfAll, which contains a more comprehensive view of the package database. This field is lazily initialized (so this means we're retaining the package database list, but this should be fine because we're already maintaining the entries of the list.) Additionally, the full view doesn't keep track of a boolean toggle for visibility/exposure anymore, but instead tracks the *provenance* of how the module binding came to be (the ModuleOrigin data type). Additionally, we move the logic for determining if a module is exposed or not from Finder.lhs and put it in Packages.lhs; this information is communicated via the LookupResult data type. Unfortunately, we can't directly return a FindResult, because this data type is defined in HscTypes which depends on Packages. This is going to change some more in the near future when I add thinning/renaming to package flags; the error messages will need to be more flexible. I've also slightly changed the semantics of error messages for package qualified imports. Previously, if we didn't find any package qualified imports, but there were hidden modules in a *different* package, the error message would prefer mentioning those as opposed to providing suggestions. Now, if a module is hidden but in the wrong package, we won't mention it; instead, it will get mentioned with the other module suggestions. I was too lazy to write a test, but I can add one if people would like. The module reexport bug was, package q reexported p:P as Conflict, and package r reexported p:P2 as Conflict, this was *not* reported as a conflict, because the old logic incorrectly decided that P and P2 were the same module on account of being from the same package. The logic here has been corrected. Contains haddock submodule update. Signed-off-by: Edward Z. Yang <firstname.lastname@example.org>