version_string() and semantic_version() etc is slow

Problem
^^^^^^^

If a particular package is *not* installed (with pip) just cloned, the version_string() (and similar methods) are really slow.

Reproduce:
^^^^^^^^^^

Choose an arbitrary python package, which uses pbr. I tested with scrapd and wexpect.

Type following commands into Windows' cmd:

```console
mkdir slow_pbr
cd slow_pbr
python -m virtualenv slow_pbr_virtualenv
slow_pbr_virtualenv\Scripts\activate
git clone https://github.com/scrapd/scrapd.git
cd scrapd
python -m pip install -r requirements.txt

python
# The following commands are python commands:
import scrapd
from pbr.version import VersionInfo
a = VersionInfo('scrapd')
a.version_string() # <--- This command is really slow. It takes several secs.
```

Here is my full transcript:

```console
Microsoft Windows [Version 10.0.18363.657]
(c) 2019 Microsoft Corporation. All rights reserved.

C:\Users\usr>mkdir slow_pbr

C:\Users\usr>cd slow_pbr

C:\Users\usr\slow_pbr>python -m virtualenv slow_pbr_virtualenv
Using base prefix 'C:\\Program Files\\Python37'
New python executable in C:\Users\usr\slow_pbr\slow_pbr_virtualenv\Scripts\python.exe
Installing setuptools, pip, wheel...
done.

C:\Users\usr\slow_pbr>slow_pbr_virtualenv\Scripts\activate

(slow_pbr_virtualenv) C:\Users\usr\slow_pbr>git clone https://github.com/scrapd/scrapd.git
Cloning into 'scrapd'...
remote: Enumerating objects: 190, done.
remote: Counting objects: 100% (190/190), done.
remote: Compressing objects: 100% (89/89), done.
remote: Total 1927 (delta 119), reused 139 (delta 95), pack-reused 1737
Receiving objects: 100% (1927/1927), 905.79 KiB | 2.58 MiB/s, done.
Resolving deltas: 100% (1278/1278), done.

(slow_pbr_virtualenv) C:\Users\usr\slow_pbr>python -m pip install -r scrapd\requirements.txt
Collecting aiohttp==3.6.1
  Downloading aiohttp-3.6.1-cp37-cp37m-win_amd64.whl (617 kB)
     |████████████████████████████████| 617 kB 1.7 MB/s
Collecting beautifulsoup4==4.8.1
  Downloading beautifulsoup4-4.8.1-py3-none-any.whl (101 kB)
     |████████████████████████████████| 101 kB 5.7 MB/s
Collecting click==7.0
  Using cached Click-7.0-py2.py3-none-any.whl (81 kB)
Collecting dateparser==0.7.2
  Downloading dateparser-0.7.2-py2.py3-none-any.whl (352 kB)
     |████████████████████████████████| 352 kB ...
Collecting loguru==0.3.2
  Downloading loguru-0.3.2-py3-none-any.whl (44 kB)
     |████████████████████████████████| 44 kB 770 kB/s
Collecting pbr>=5.2.0
  Using cached pbr-5.4.4-py2.py3-none-any.whl (110 kB)
Collecting pydantic==0.32.2
  Downloading pydantic-0.32.2-py36.py37.py38-none-any.whl (63 kB)
     |████████████████████████████████| 63 kB 4.5 MB/s
Collecting tenacity==5.1.1
  Downloading tenacity-5.1.1-py2.py3-none-any.whl (34 kB)
Collecting async-timeout<4.0,>=3.0
  Downloading async_timeout-3.0.1-py3-none-any.whl (8.2 kB)
Collecting yarl<2.0,>=1.0
  Downloading yarl-1.4.2-cp37-cp37m-win_amd64.whl (125 kB)
     |████████████████████████████████| 125 kB ...
Collecting attrs>=17.3.0
  Downloading attrs-19.3.0-py2.py3-none-any.whl (39 kB)
Collecting chardet<4.0,>=2.0
  Using cached chardet-3.0.4-py2.py3-none-any.whl (133 kB)
Collecting multidict<5.0,>=4.5
  Downloading multidict-4.7.5-cp37-cp37m-win_amd64.whl (48 kB)
     |████████████████████████████████| 48 kB ...
Collecting soupsieve>=1.2
  Downloading soupsieve-2.0-py2.py3-none-any.whl (32 kB)
Collecting regex
  Downloading regex-2020.2.20-cp37-cp37m-win_amd64.whl (271 kB)
     |████████████████████████████████| 271 kB 6.4 MB/s
Collecting tzlocal
  Downloading tzlocal-2.0.0-py2.py3-none-any.whl (15 kB)
Collecting python-dateutil
  Downloading python_dateutil-2.8.1-py2.py3-none-any.whl (227 kB)
     |████████████████████████████████| 227 kB ...
Collecting pytz
  Using cached pytz-2019.3-py2.py3-none-any.whl (509 kB)
Collecting colorama>=0.3.4; sys_platform == "win32"
  Using cached colorama-0.4.3-py2.py3-none-any.whl (15 kB)
Collecting win32-setctime>=1.0.0; sys_platform == "win32"
  Downloading win32_setctime-1.0.1-py3-none-any.whl (3.4 kB)
Collecting six>=1.9.0
  Using cached six-1.14.0-py2.py3-none-any.whl (10 kB)
Collecting idna>=2.0
  Downloading idna-2.9-py2.py3-none-any.whl (58 kB)
     |████████████████████████████████| 58 kB 2.7 MB/s
Installing collected packages: async-timeout, idna, multidict, yarl, attrs, chardet, aiohttp, soupsieve, beautifulsoup4, click, regex, pytz, tzlocal, six, python-dateutil, dateparser, colorama, win32-setctime, loguru, pbr, pydantic, tenacity
Successfully installed aiohttp-3.6.1 async-timeout-3.0.1 attrs-19.3.0 beautifulsoup4-4.8.1 chardet-3.0.4 click-7.0 colorama-0.4.3 dateparser-0.7.2 idna-2.9 loguru-0.3.2 multidict-4.7.5 pbr-5.4.4 pydantic-0.32.2 python-dateutil-2.8.1 pytz-2019.3 regex-2020.2.20 six-1.14.0 soupsieve-2.0 tenacity-5.1.1 tzlocal-2.0.0 win32-setctime-1.0.1 yarl-1.4.2

(slow_pbr_virtualenv) C:\Users\usr\slow_pbr>cd scrapd

(slow_pbr_virtualenv) C:\Users\usr\slow_pbr\scrapd>python
Python 3.7.5 (tags/v3.7.5:5c02a39a0b, Oct 15 2019, 00:11:34) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import scarpd
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'scarpd'
>>> import scrapd
>>> from pbr.version import VersionInfo
>>> a =VersionInfo('scrapd')
>>> a.version_string()
'3.0.5'
```

I use:
^^^^^^
 - Win 10.0.18363 Build 18363
 - Python 3.7.5
 - pbr 5.4.4