git-annex.cabal: Deal with breaking changes in Cabal 2.0

https://github.com/haskell/cabal/issues/4655

This means that when a module is conditionally imported via ifdef
depending on the OS or build flags, the cabal file has to mirror the
same logic there to only list the module then.

Since there are lots of OS's and lots of combinations of build flags
here, it's rather difficult to know if the cabal file has been completelty
correctly updated to match the source code.

So I am very unhappy with needing to update things in two places. I've
only tested this on linux with most build flags enables; this will
probably need significant time and testing to catch every cabal file
tweak that this change to Cabal requires. And it will be a continual
source of compile failures going forward when the code is modified and
the cabal file not also updated.

DRY DRY DRY, I repeat myself, but: DRY! Sigh..

(Also, had to remove all Build.* that are standalone programs from the
Other-Modules list, because since cabal passes those modules to ghc when
building git-annex, it complains that they use module Main. Those
modules are only used when building with the Makefile anyway, so this
change shouldn't break anything.)

This commit was sponsored by Thomas Hochstein on Patreon.

doc/bugs/Cannot_build_with_GHC_8.2.1/comment_6_e948c8fcf34559808726aaa0d5f5f020._comment

@@ -0,0 +1,14 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 6"""
+ date="2017-08-18T15:02:42Z"
+ content="""
+I've updated the cabal file, it should build now. Although unfortunately
+the breaking changes to cabal are such that it might still fail to build
+with some OS's and some combinations of build flags, where it used to build
+before. I only tested on Linux with stack (modified for resolver:
+nightly-2017-08-17)
+
+As to the quickcheck failures, I think that's the same problem I already
+fixed in [[!commit da8e84efe997fcbfcf489bc4fa9cc835ed131d3a]].
+"""]]