refactor dependency specification parsing logic

[abn]

This change moves, cleans up and refactors dependency specification parsing logic from InitCommand to poetry.utils.dependency_specification. This is done to improve usability and maintainability of this logic.

This has been bugging me for a while now, so decided to clean it up a bit.

Additionally, with this change, users can now add dependencies using valid PEP 508 strings.

$ poetry add 'cachecontrol[filecache] (>=0.12.9,<0.13.0); python_version >= "3.6"' --lock

Updating dependencies
Resolving dependencies... (0.5s)

Writing lock file
$ cat pyproject.toml | grep -i cachecontrol
CacheControl = {version = ">=0.12.9,<0.13.0", markers = "python_version >= \"3.6\"", extras = ["filecache"]}

[neersighted]

This is a whole lot cleaner and easier to read -- nice work!

[github-actions]

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