Reset imported src files on each call to flatten

[prathamesh0]

Part of cerc-io/watcher-ts#23

• Reset imported src files on each call to flatten:
  • Earlier, on multiple calls to the exported function flatten, if the input contracts had common imports, they would be placed in the output of just a single contract and not all the contracts where they are used. The issue was caused due to usage of a global variable (importedSrcFiles) to store the imported files.
  • Now that the variable to store the imported source files is reset on each call to flatten a contract, the common imports are output for every contract that imports them.