Replace updates of os.environ with explcit passing of `env` to
subprocess calls.

Relates-to: #3199

Resolves: #3303

This change allows users to run embedded pip via `poetry run pip`.

Use embedded pip wheel from virtualenv package. This avoids the need
for pip to be a critical package in the project's virtual environment.

In some cases, legacy repositories might not provide a checksum as a
url fragment or use a deprecated algorithm. In these scenarios, this
change ensures that poetry downloads and calculates the sha256 checksum
for the file.

Resolves: #1631 #1553

This change ensures that when exporting `requirements.txt`, local 
direct reference dependencies are exported as uri and not paths.

Resolves: #3189

This reverts commit 37b36f48 as this is no longer required.

I think the "Expanded dependency specification syntax" paragraph added in
commit c94c34c8 was inserted in the wrong place
and cut the note from its corresponding block.

Fixes #2867, the virtual environment found seems to be broken
messages when running in MSYS2. This change detects when in MSYS2
in order to use the bin directory for the virtualenv instead of
Scripts which is normally used in Windows.

This repairs a broken triple-bang note.  Also cleans up a few stray double-newlines and very long lines to match rest of file.

Resolves: #1393

Co-authored-by: wyl8899 <wyl8899k@gmail.com>
Co-authored-by: vlad0337187 <naumovvladislav@mail.ru>