pkg: import git dynamically

This allows dvc to work even if there is no git present, as long as user
is not using `dvc pkg` command directly.

Fixes #1835

Signed-off-by: Ruslan Kuprieiev <[email protected]>

dvc/repo/pkg.py

@@ -2,7 +2,6 @@ from __future__ import unicode_literals
 
 import os
 import shutil
-from git.cmd import Git
 
 import dvc.logger as logger
 from dvc.exceptions import DvcException
@@ -60,6 +59,8 @@ class GitPackage(Package):
     DEF_DVC_FILE_PREFIX = "mod_"
 
     def install_or_update(self, parent_repo, pkg_params):
+        from git.cmd import Git
+
         if not self.is_in_root():
             raise DvcException(
                 "This command can be run only from a repository root"

dvc/temp_git_repo.py

@@ -2,8 +2,6 @@ import os
 import shutil
 import tempfile
 
-import git
-
 from dvc import logger
 from dvc.exceptions import DvcException
 
@@ -70,6 +68,8 @@ class TempGitRepo(object):
         return self
 
     def _clone_to_temp_dir(self):
+        import git
+
         result = tempfile.mktemp(
             prefix=self._tmp_mod_prefix, dir=self.modules_dir
         )