improve install_files for binaries

[rurban]

Currently the logic to install installables is not satisfactory and leads too easily to hard to detect errors.

There's no error when a installable_ version is missing.

Currently:
First the ordinary binaries are installed, then in a second step those are overwritten by the installable_* versions.

Todo:

• For all binaries with matching installable_* binary, copy only the installable_ binary and do not attempt to copy the build_specific variant at all. This is too fragile.
• print a warning on missing installable version if not found in the MANIFEST files (such as scripts)
• fail with an error if the installable is missing or could not be copied.