pickle: replace compilers with those in the coredata

By storing an environment in the compilers, commit a6c4a4d introduced
a link from coredata.dat to the Environment object.  This creates a
circular link between build.dat and coredata.dat.  Until now, the
circular link caused build.dat to store separate copies of the
compilers, and potentially through those compilers even a different
Environment and CoreData (!).

The previous patch fixed all references from Compiler to Environment,
by storing a proxy to the Environment in coredata.dat.  However,
there is still the problem that targets can refer to compilers that
are different from those in the CoreData, resulting in incorrect
introspection data.  Fix this by storing proxies from the Build
to the compilers in the coredata.

This creates a cycle because the Environment is needed to load the
CoreData, and Build (which contains the Environment) needs the CoreData
to be loaded first.  To break it, first load the Environment minus the
coredata, then the CoreData, then the Build.

A much cleaner solution would be to make callers of build.load() use
either the introspection data or (as a last resort) the coredata; and
remove altogether the need to pickle the Build and Environment.  But that
is a bigger change, and even if that is done, the need for MesonPickler (to
avoid new occurrences of the bug) and CoreDataPickler remains.  Only this
commit and "pickle: extract checks on loaded objects" could be reverted.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>