Reduce path lengths for temp package src and build dirs

This is for paths under $dist/tmp/ where we unpack package source
tarballs and build them. This is for packages destined for the store.
The names of these tmp dirs don't matter since they're guaranteed to be
unique, so we only need short identifiers. So stop using full package
names, and put the build artifacts in their own dir, rather than using a
dist dir under the src dir.

Author: dcoutts

cabal-install/Distribution/Client/ProjectBuilding.hs

@@ -743,15 +743,16 @@ withTarballLocalDirectory verbosity distDirLayout@DistDirLayout{..}
                           tarball pkgid dparams buildstyle pkgTextOverride
                           buildPkg  =
       case buildstyle of
-        -- In this case we make a temp dir, unpack the tarball to there and
-        -- build and install it from that temp dir.
+        -- In this case we make a couple temp dirs, unpack the tarball to one
+        -- and build and install it from the other. We avoid nesting the
+        -- builddir under the tarball src dir to keep path name lengths down.
         BuildAndInstall ->
-          withTempDirectory verbosity distTempDirectory
-                            (display (packageName pkgid)) $ \tmpdir -> do
-            unpackPackageTarball verbosity tarball tmpdir
+          let tmpdir = distTempDirectory in
+          withTempDirectory verbosity tmpdir "src"   $ \unpackdir ->
+          withTempDirectory verbosity tmpdir "build" $ \builddir -> do
+            unpackPackageTarball verbosity tarball unpackdir
                                  pkgid pkgTextOverride
-            let srcdir   = tmpdir </> display pkgid
-                builddir = srcdir </> "dist"
+            let srcdir   = unpackdir </> display pkgid
             buildPkg srcdir builddir
 
         -- In this case we make sure the tarball has been unpacked to the