Apply 'sandboxInstallPolicy' after 'reinstallTargets'.

Otherwise 'install --reinstall' in a sandbox reinstall the contents of the whole
sandbox.
Set 'haddockDistPref' to the right value when working inside a sandbox.

Fixes #1332.
Remove an obsolete comment.
Make newly-added add-source deps override previously installed versions.

Fixes #1197.

This patch is a bit large because it includes several related changes:

1) Remove 'installUseSandbox' from 'InstallFlags' and pass 'useSandbox' as an
additional argument instead.

2) Instead of calling 'reinstallAddSourceDeps' from 'installAction', always pass
'SandboxPackageInfo' to 'install'.

3) Set the timestamps of newly-added add-source deps to 0 in the timestamp file.

4) Move the timestamp file update to 'postInstallActions' from
'withModifiedDeps'. This way, the timestamps are updated even when the user runs
'install --only-dependencies' or 'install some-add-source-dep-package-id'.
Merge pull request #1331 from 23Skidoo/new-add-source-deps-have-priority

Make newly-added add-source deps override previously installed versions.
Don't update 'cabal.sandbox.config' after 'configure -w'.

Use the values from 'ConfigFlags' that were read from 'dist/setup-config'
instead.

This way 'reinstallAddSourceDeps' won't reinstall deps for the wrong compiler,
but the user will be forced to use 'install -w' when installing inside a
sandbox. This is consistent with the normal mode of operation.

Fixes #1333, in a way.
Expand a comment.
Merge pull request #1334 from 23Skidoo/no-with-compiler-sandbox-config

Don't update `cabal.sandbox.config` after `configure -w`.
Improve update performance, see issue #799
Display a more user friendly message.

When the local and remote repositories match.
Document etag parameter for mkRequest & mkRequest.
Continue with program execution when we get a 304.
Use notice instead of putStrLn.
Make 304 user message more generic.
Fix the punctuation.
Avoid extracting a cached repository.
Use a sum type as the return for downloadIndex.
Use DownloadResult in downloadURI.
Merge pull request #1341 from gostrc/master

Improve update performance (#799)
New helper function: removeExistingFile.