command/new: support namespace package creation

With this change, names containing "." are treated as namespace
packages.

The following behaviour is now expected.

- "a.b.c" creates package name "a-b-c" with directory structure
  "a/b/c"
- "a-b-c" creates package name "a-b-c" with directory structure
  "a/b/c"
- "a.b_c" creates package name "a-b-c" with directory structure
  "a/b_c"
- "a_b_c" creates package name "a-b-c" with directory structure
  "a_b_c"