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'.