`rustc_span::symbol` defines some things that are re-exported from
`rustc_span`, such as `Symbol` and `sym`. But it doesn't re-export some
closely related things such as `Ident` and `kw`. So you can do `use
rustc_span::{Symbol, sym}` but you have to do `use
rustc_span::symbol::{Ident, kw}`, which is inconsistent for no good
reason.
This commit re-exports `Ident`, `kw`, and `MacroRulesNormalizedIdent`,
and changes many `rustc_span::symbol::` qualifiers in `compiler/` to
`rustc_span::`. This is a 200+ net line of code reduction, mostly
because many files with two `use rustc_span` items can be reduced to
one.
This moves some code around and adds some documentation comments to make
it easier to understand what's going on with the entrypoint logic, which
is a bit complicated.
The only change in behavior is consolidating the error messages for
unix_sigpipe to make the code slightly simpler.
In fc357039f9 `#[main]` was removed and replaced with `#[rustc_main]`.
In some place the rename was forgotten, which makes the current code
confusing, because at first glance it seems that `#[main]` is still
around. Perform the renames also in these places.
