`flutter create` should fail if called from inside the flutter repo

[Hixie]

We saw this happen in the usability study today (P2), and it has some subtle negative effects later, for example they ended up having the wrong set of analyzer warnings.

[sethladd]

Should we not allow someone to create a new project inside a non-empty directory? Seems like the safe thing to do.

[abarth]

Should we not allow someone to create a new project inside a non-empty directory? Seems like the safe thing to do.

Some error messages recommend doing that to restore missing files.

[github-actions]

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