Read package name from pyproject.toml

[cjolowicz]

• Add Poetry.config.name property to provide access to the package name as defined in pyproject.toml.
• Use this property to construct the egg fragment for sdist file URLs.

Rationale:

• Avoid spawning a subprocess running poetry version to determine the name.
• Avoid the need to parse Poetry output meant for human consumption.
• We will need to parse pyproject.toml anyway for extras support.

Note that poetry version canonicalizes the package name. We don't bother here, because pip uninstall will do so anyway.

This adds a core dependency on tomlkit, the TOML parser used by Poetry.