Description of problem:
"mbs-build local" needs the mockbuild.scm Python package which is in the mock-scm RPM package, but there's no dependency on it.

Version-Release number of selected component (if applicable):
module-build-service-1.3.24-2.fc26.noarch

How reproducible:
Reproducible

Steps to Reproduce:
1. "mbs-build local"

Actual results:
...
ERROR: local variable 'mockbuild' referenced before assignment
Traceback (most recent call last):
  File "/usr/libexec/mock/mock", line 714, in run_command
    import mockbuild.scm
ImportError: No module named 'mockbuild.scm'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/libexec/mock/mock", line 886, in <module>
    main()
  File "/usr/lib/python3.5/site-packages/mockbuild/trace_decorator.py", line 89, in trace
    result = func(*args, **kw)
  File "/usr/libexec/mock/mock", line 701, in main
    run_command(options, args, config_opts, commands, buildroot, state)
  File "/usr/lib/python3.5/site-packages/mockbuild/trace_decorator.py", line 89, in trace
    result = func(*args, **kw)
  File "/usr/libexec/mock/mock", line 716, in run_command
    raise mockbuild.exception.BadCmdline(
UnboundLocalError: local variable 'mockbuild' referenced before assignment
...

Expected results:
Module builds successfully.