[Rewrite] Creating a Python 2.7 environment does not copy python27.dll

[pfmoore]

To reproduce, start with an environment with no Python 2.7 installed.

• Get the 2.7.17 MSI installer from python.org and run it.
• Select the "install just for me" option (the default option to install globally puts python27.dll in the windows directory, which masks the bug).
• Choose all defaults, but deselect "Register extensions" and "Add Python to PATH".
• C:\Python27\python.exe -m pip install virtualenv-rewrite.whl (I built the rewrite branch to a local wheel file and installed that, you can probably install the rewrite branch any way you like).
• C:\Python27\python.exe -m virtualenv xx

Now look in xx\Scripts and there is no python27.dll file. Running xx\Scripts\python.exe fails with an error "python27.dll not found".

[gaborbernat]

Yeah, this seems like something we need to fix; I'll tackle this and other feedback issues after #1482, which turned out a lot harder than expected (pypy unicode support with files has lot of gaps compared to CPython 😢) Thanks!