Prevent accidental creation of nested projects

[abravalheri]

Purpose

This PR attempts to prevent users from accidentally creating nested projects, which may result in problems with setuptools-scm, as described in #544.

Approach

The pyscaffold.actions.verify_project_dir was modified to also look for nested repositories and raise an error when relevant.

If the --force option is given, PyScaffold will understand that the users know what they are doing and create the project without complaining.

[abravalheri]

@FlorianWilhelm, is this more or less what you had in mind?

[coveralls]

Pull Request Test Coverage Report for Build 6720378509721600

• 11 of 11 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 97.439%

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Totals
Change from base Build 5014187148574720:	0.01%
Covered Lines:	1678
Relevant Lines:	1711

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💛 - Coveralls

[FlorianWilhelm]

Wow! You are so fast 🚀 Looks really good :-)

BTW, I noticed yesterday again how fast the unit tests are. Thanks again for having sped them up so much ⚡