Less import overhead for errors
This removes huge (3+ lines) import lists found in files that had their error reporting migrated. These lists are bad for developer workflows as adding, removing, or editing a single error's name might cause a chain reaction that bloats the git diff. As the error struct names are long, the likelihood of such chain reactions is high.
Follows the suggestion by `@Nilstrieb` in the [zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/147480-t-compiler.2Fwg-diagnostics/topic/massive.20use.20statements) to replace the `use errors::{FooErr, BarErr};` with `use errors;` and then changing to `errors::FooErr` on the usage sites.
I have used sed to do most of the changes, i.e. something like:
```
sed -i -E 's/(create_err|create_feature_err|emit_err|create_note|emit_fatal|emit_warning)\(([[:alnum:]]+|[A-Z][[:alnum:]:]*)( \{|\))/\1(errors::\2\3/' path/to/file.rs
```
& then I manually fixed the errors that occured. Most manual changes were required in `compiler/rustc_parse/src/parser/expr.rs`.
r? `@compiler-errors`
bors
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