This function always returns the value from sys_crate_name.

`crate::ffi`, which these calls resolve to are ignored in
Imports::common_checks, no need to pass these explicitly.

This is used in enum function generation where all enum types
asssociated with a given crate are defined in the same file.

This not only cleans up unnecessary mutable variables in the codegen
loop and a cloned Imports instance (so that it can remain immutable
during codegen), it also adds the missing rejection logic to enum
analysis.  No harm was caused because these checks happened at codegen
prior, but they omit some unnecessary imports.

This function is responsible for generating a Display impl, and caused
unused std::fmt import previously.

Analysis was copied from enums which adds an std::fmt when
generate_display_trait is true, but the codegen for it was missing.
Instead of removing the import, provide a simple Display trait that
forwards to bitflags' Debug implementation.

When get_name or to_string is available and adheres to the right
conditions a Display impl leveraging this C implementation is generated.
This will clash with a pure Rust impl if generate_display_trait is true
which should be disabled to prefer the C function call instead.

Complements: 5ce31f2 ("codegen: Add version condition on special function traits")