Improve the add and init commands

[sdispater]

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

This PR improves the add and init commands in the following ways.

Easier addition of git and path dependencies

It's now easier to add git and path dependencies, and they can now be added at the same time as other packages:

poetry add requests git+https://github.com/sdispater/pendulum.git
poetry add git+https://github.com/sdispater/pendulum.git@develop
poetry add ./my-package/
poetry add ../my-package/dist/my_package-0.1.0.whl

It also removes the --git and --path options since they are no longer needed.

Since the logic is shared, init now supports adding git and path dependencies and you get a new message before adding your first package:

You can specify a package in the following forms:
  - A single name (requests)
  - A name and a constraint (requests ^2.23.0)
  - A git url (https://github.com/sdispater/poetry.git)
  - A git url with a revision (https://github.com/sdispater/poetry.git@develop)
  - A file path (../my-package/my-package.whl)
  - A directory (../my-package/)

Bump the constraint of an existing depency

You can now update the constraint of an already present dependency (which would raise an error previously) by specifying a specific constraint or by using the special latest constraint:

poetry add pendulum@^2.0.5
poetry add pendulum@latest

Better handling of prereleases

Now, if a package only has prereleases, the highest one will be selected. Before this change, the command would return a message saying the package didn't have any version matching the constraint.

$ poetry add black --dev
Using version ^19.3b0 for black

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