add support for git submodules

Git submodules allow to keep a git repository as a subdirectory of another git repository. This allows to clone another repository into a project and keep commits separate.
Terra appears to require at the moment that all wdl scripts for a wdl workflow are in one git repository.
In the spirit of following good software practice and not duplicate code, i.e. make code more reusable, Terra should support git submodules.

Currently, an error is generated during the attempt to import via dockstore a wdl workflow from a git repository with git submodules. For example:

Error importing workflow
Failed to import './create_fastq_qc_report/task_create_fastq_qc_report.wdl' (reason 1 of 1): Failed to resolve './create_fastq_qc_report/task_create_fastq_qc_report.wdl' using resolver: 'http importer (relative to https://raw.githubusercontent.com/ranlib/fastq_qc/master/)' (reason 1 of 1): Failed to download https://raw.githubusercontent.com/ranlib/fastq_qc/master/create_fastq_qc_report/task_create_fastq_qc_report.wdl (reason 1 of 1): 404: Not Found