Fix the common/shared source tree layout inconsistency. If it goes in
libmircommon then it should be called "common" and not "shared":

  src/client/, include/client/ -> libmirclient, include/mirclient/
  src/platform/, include/platform/ -> libmirplatform, include/mirplatform/
  src/server/, include/server/ -> libmirserver, include/mirserver/
  src/shared/, include/shared/ -> libmircommon, include/mircommon/
has changed to:
  src/common/, include/common/ -> libmircommon, include/mircommon/

Approved by camako, afrantzis, kdub. Merged manually to work around
unrelated CI failures.