Use 'spec_string' for ReqSpec original string name.

Previously it was 'req_spec_string' to distinquish
from RepositorySpec, but RepoSpec and ReqSpec are
separated now, so just use 'spec_string' for both
as that is what the repo spec parsing code does.

Add 'scm' attribute back to RequirementSpec.

Fixes part of #236

Fixes "AttributeError: 'RequirementSpec' object has no attribute 'scm'"
error on 'mazer install git+https://github.com/alikins/whatever'

(collection install still fails, but for a different reason)

ansible_galaxy/models/requirement_spec.py

@@ -27,7 +27,10 @@ class RequirementSpec(object):
     version_aka = attr.ib(default=None)
     fetch_method = attr.ib(default=None, cmp=False)
     src = attr.ib(default=None, cmp=False)
-    req_spec_string = attr.ib(default=None, cmp=False)
+    scm = attr.ib(default=None, cmp=False)
+
+    # If created from a parsed string, spec_string is copy of the full string
+    spec_string = attr.ib(default=None, cmp=False)
 
     @property
     def label(self):
@@ -58,7 +61,8 @@ def from_dict(cls, data):
                        version_spec=version_spec_str,
                        version_aka=data.get('version_aka', None),
                        fetch_method=data.get('fetch_method', None),
-                       req_spec_string=data.get('req_spec_string', None),
+                       scm=data.get('scm', None),
+                       spec_string=data.get('spec_string', None),
                        src=data.get('src', None),
                        )
         return instance

ansible_galaxy/requirements.py

@@ -15,7 +15,6 @@ def from_dependencies_dict(dependencies_dict, namespace_override=None, editable=
                                               namespace_override=namespace_override,
                                               editable=editable)
         req_spec_data['version_spec'] = req_version_spec
-        req_spec_data['req_spec_string'] = req_spec_data.pop('spec_string', None)
 
         log.debug('req_spec_data: %s', req_spec_data)