Add validation for crate/package name in new/init

[alonme]

solves https://github.com/PyO3/maturin/issues/1398

The current commit only adds a very basic validation as a POC before implementing the needed cargo / pypi checks A few questions i have before finishing the implementation

1. The cargo validation seems to be comprised of 2 main parts, a. Basic validation which is part of the crate cargo-util-schemas, and is not made public outside the crate

   b. Extra validations that are public from the cargo package

   I believe we should vendor these logics - one is private, and the other requires the cargo package which i believe will add many dependencies to the project

2. The generate_project function is de-facto defining a default value for GenerateProjectOptions.name - would it be better to move this logic into GenerateProjectOptions?

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[messense]

1. I believe we should vendor these logics - one is private, and the other requires the cargo package which i believe will add many dependencies to the project

Agreed.

2. The generate_project function is de-facto defining a default value for GenerateProjectOptions.name - would it be better to move this logic into GenerateProjectOptions?

Makes sense.

[alonme]

@messense I believe this is ready for review 😄

[alonme]

@messense Thanks!