The set of SFDX source commands has grown to include (presently beta):
sfdx force:source:deploy
and that seems similar to the mdapi commands:
sfdx force:mdapi:deploy
What is different?
(In particular, it would be great to be able to directly deploy code in SFDX format to a namespace org used for managed package uploading without having to convert to MDAPI format first, but I assume that is still not possible)