fix(factory): safe get extras in scripts and config

[mkniewallner]

Resolves: python-poetry/poetry#4665

• Added tests for changed code.
• Updated documentation for changed code. Not applicable

According to:

poetry-core/src/poetry/core/json/schemas/poetry-schema.json

Lines 6 to 10 in 3bf7ad0

	"required": [
	"name",
	"version",
	"description"
	],

poetry-core/src/poetry/core/json/schemas/poetry-schema.json

Lines 597 to 600 in 3bf7ad0

	"required": [
	"reference",
	"type"
	]

• extras is not required in the main config (under poetry.tools.extras)
• extras is not required when defining a script (under poetry.tools.scripts) using the syntax introduced in Added script file feature #40

Despite that, Factory.validate assumes that extras key is always present for both cases by hard accessing it, making the script definition fail for 2 possible reasons:

• poetry.tool.extras is not defined at all
• a script defined as a dict doesn't define an extras key

This PR fixes those 2 use cases by soft accessing the key and defaulting to an empty list/dict.
It also adds tests to validate the errors/warnings that could happen when using the strict flag, since it was lacking some coverage.

[mkniewallner]

The tests that is failing does not seem related to the changes in the PR.
I can also reproduce locally and on main branch, and only on Python 3.7, same as the CI.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

No Coverage information
0.0% Duplication

[mkniewallner]

Rebased from main to pick up #406 so that tests can pass.