* Add context to caret version constraint documentation

Resolves: #3272

Resolves: #3263

When a project specifies non default sources, PyPI gets added as the
default source. This will prioritise packages available in PyPI when
the package exists in both index. This change ensures that PyPI is
only used as a default when no other sources are provided.

Resolves: #1677 #2564 #3238

Resolves: #3213

Previously, when using locked repository, incorrect dependency instance
was created when a dependency's extra requirement activated a
nested extra. This change ensures that these are correctly
loaded.

As part of this change new lock files write PEP 508 serialised form of
extra dependencies in order to reuse core logic to parse specification
of extra requirement.

Resolves: #3224

When detecting installed packages, this change ensures that packages
with .pth files are not incorrectly marked as editable. A package is
considered editable only if at least one of the paths detected is
not in the environment site.

Resolves: #3077

This change ensures that, When using system environment, poetry falls
back to `userbase` if default location is not writable.

This change improves handling of site-packages under system env, by
gracefully handling fallbacks to user site when required and possible.

Resolves: #3079

This change allows users to disable parallel execution while using the
new installer.

Resolves: #3087

Resolves: #1894

This change correctly reverts pyproject.toml changes for dry runs as 
well as keyboard interrupts.

Resolves: #2933 #318

Mitigates: #3134

This change drops support for Python 2.7 and 3.5 in tox, ci and release
workflows in preparation for dropping support for these versions in
Poetry 1.2.0.

(cherry picked from commit ed43d94b423a7bf53dfb530a549b686d016ab891)

(cherry picked from commit 733736cd1c2ea6189a77e34ab640059dbff2be23)

Previously, when determining nested dependencies, the check for
activated extras/features of top level dependencies were done after the
nested dependencies were processed. This lead to exports containing
in active extras. This change resolves this by pre-selecting top level
packages prior to identifying nested dependencies.

(cherry picked from commit 2b6f82e9494b99bdb04d0dac90f87eb23548e4a5)

(cherry picked from commit a7d66767a089c5604803a0a2d96ddc5fcb41c043)

(cherry picked from commit b29450c19034cf34856cf6e2a610507411833e36)

This change ensures that markers are propagated from top level
dependencies to the deepest level by walking top to bottom instead of
iterating over all available packages.

In addition, we also compress any dependencies with the same name and
constraint to provide a more concise representation.

Resolves: #3112 #3160
(cherry picked from commit e78a67ba139c70ee1856834711ddaf14de0c926a)

Resolves: #3115
(cherry picked from commit 04967db7ab20558b593e6b47ef5258e95a1b642f)

This change, moves common functionality from exporter into the locker
to allow for sharing of logic that can generate `DependencyPackage`
instances given a a list of requirements using the lock data.

(cherry picked from commit c2adb3283c0b76e290a8f25053bf035500b47cb2)

Resolves: #3116
(cherry picked from commit 50e06289a40819afe68f9b9cfecd7ccf08763476)

Resolves: #3129
(cherry picked from commit 38fddf05b36ec64647e66bf459578f9e0b41442d)

Resolves: #3069

Relates-to: #3155

Closes #3144