Allow Rust crate to be placed outside of the directory containing pyproject.toml

[messense]

Lifting the two limitations in

1. maturin/src/project_layout.rs

   Line 242 in 0ed2757

   bail!("Cargo.toml can not be placed outside of the directory containing pyproject.toml");

2. maturin/src/source_distribution.rs

   Lines 356 to 359 in 0ed2757

   	bail!(
   	"pyproject.toml was not included by `cargo package`. \
   	Please make sure pyproject.toml is not excluded or build without `--sdist`"
   	)

It'd be useful for projects using the cffi bindings, to have the c-abi crate and python bindings separated, for example

py/
  pyproject.toml
cabi/
  Cargo.toml