Add a simplified freshness check for dependencies.

[ta0kira]

Currently, if a dependency isn't out of date, but it's compile-metadata is newer than the module depending on it, the latter will be considered out of date. But, it should be fine if the public .0rp sources from the dependency aren't newer than the module depending on them.

Here is an example situation:

• lib/file depends on lib/util.
• A user-written module foo depends on lib/file.
• Someone updates only the C++ code for lib/util and recompiles just that module with zeolite -r.
• lib/util itself isn't considered out of date, but now lib/file is, thereby preventing compilation of foo.

It would be helpful if the freshness check for foo used a simplified freshness check for lib/file that only includes the .0rp files, since recompiling lib/file in incremental mode won't be affected by implementation changes to lib/util.

So, to check a module's freshness:

• Keep the existing checks that cover all input and output files.
• For each dependency (provided it's up to date), check the modification times of each .0rp file against the modification time of the depending module's compile-metadata. (The freshness check of the dependency will ensure that all .0rp files are listed in its compile-metadata.)

[ta0kira]

This won't work without removing the Namespace in category deps (in object_files) that come from other modules, because recompiling a dep will invalidate its namespaces.

For example, non-local category deps should be changed to NoNamespace in resolveObjectDeps, and getObjectFileResolver should replace NoNamespace with the owning module's public_namespace.

[ta0kira]

I take that back: Linking fails because compiled .cpp sources depend on an outdated C++ namespace. So, this isn't feasible at the moment.

[ta0kira]

Actually, it should be fine to make the public namespace deterministically based on absolute path and compiler hash.

[ta0kira]

If making the public namespace deterministic is necessary, then that obviates clearing/setting the Namespace in object_files.

[ta0kira]

Rather than assuming the public namespace is deterministic, the freshness check could check that the Namespace in object_files matches the public namespace of the dep.