test: pkg: add func test

[dmpetrov]

Only very basic tests were developed for pkg install. However, a fixture framework was already redesigned to cover pkg install needs - which might use multiple repositories in a single test (the old test fixtures allows only a single repo).

Test cases:

• cannot copy repository (it is missing, access issues)
• cannot pull data (access issues, not enough space on hdd)
• what if an installed file overwrites an existing one.
• dvc_mod dir exist\not
  And many others.

Today the test coverage in pkg files only:

dvc/repo/pkg.py                   96     20    79%
dvc/command/pkg.py                26      6    77%
dvc/temp_git_repo.py              91     13    86%

It needs to be at least 95%.

Note, this command designed to be atomic. If some issue happens during clone or pull then the old version of the package (in dvc_mod/mypackage dir) shouldn't be removed. See special class TempGitRepo.

[efiop]

Partially fixed by #2012