Make coverage reports only include code from the source directories

cms/.coveragerc

 # .coveragerc for cms
 [run]
 data_file = reports/cms/.coverage
+source = cms
 
 [report]
 ignore_errors = True

common/lib/capa/.coveragerc

 # .coveragerc for common/lib/capa
 [run]
 data_file = reports/common/lib/capa/.coverage
+source = common/lib/capa
 
 [report]
 ignore_errors = True

common/lib/xmodule/.coveragerc

 # .coveragerc for common/lib/xmodule
 [run]
 data_file = reports/common/lib/xmodule/.coverage
+source = common/lib/xmodule
 
 [report]
 ignore_errors = True

lms/.coveragerc

 # .coveragerc for lms
 [run]
 data_file = reports/lms/.coverage
+source = lms
 
 [report]
 ignore_errors = True