More explicit error message when initializaing in a repository with an invalid name

[qsantos]

When trying to run buffrs init in a directory with an invalid name for a package, for instance a.b, the error message does not explain that the name was automatically inferred from the name of the directory. In particular, the name is not shown.

Compare the error message of buffrs init with that of cargo init:

~/a.b% buffrs init
Error:   × failed to initialize impl package
  ╰─▶ package name must consist of only ASCII lowercase and dashes, but contains . at position 1

~/a.b% cargo init
error: invalid character `.` in package name: `a.b`, characters must be Unicode XID characters (numbers, `-`, `_`, or most letters)
If you need a package name to not match the directory name, consider using --name flag.
If you need a binary with the name "a.b", use a valid package name, and set the binary name to be different from the package. This can be done by setting the binary filename to `src/bin/a.b.rs` or change the name in Cargo.toml with: