Suggest importable type that differs slightly

[GTimothy]

This PR proposes to propose case insensitive import suggestions.
Previous discussion of this topic here: #72641 and in this PR: #72988
If that is still of interest, a discussion may be needed to limit or expand the included list.
I initially followed the suggestions in #72988.

The tests are based on the tests by @chrissimpkins in #72988.
I added a is_exact_match field to ImportSuggestion to eventually modify the suggestion text accordingly.

Only when no other suggestion is made, do I check for case insensitive import suggestion.

fn test_layout(_x: LayOut){}
//~^ ERROR: cannot find type `LayOut` in this scope

error[E0425]: cannot find type `LayOut` in this scope
  --> $DIR/libstd.rs:3:20
   |
LL | fn test_layout(_x: LayOut){}
   |                    ^^^^^^ not found in this scope
   |
help: consider importing this similarly named struct
   |
LL + use std::alloc::Layout;
   |

for a more complex case:
SystemTime exists in the stdlib, but I only suggest case insensitive import when nothing else is suggested

fn test_systemtime(_x: Systemtime){}
//~^ ERROR: cannot find type `Systemtime` in this scope

struct SystemTome{}
mod st{
    struct SystemTame{}
}

error[E0425]: cannot find type `Systemtime` in this scope
  --> $DIR/libstd.rs:428:24
   |
LL | fn test_systemtime(_x: Systemtime){}
   |                        ^^^^^^^^^^
...
LL | struct SystemTome{}
   | ----------------- similarly named struct `SystemTome` defined here
   |
help: a struct with a similar name exists
   |
LL - fn test_systemtime(_x: Systemtime){}
LL + fn test_systemtime(_x: SystemTome){}
   |

The work also checks for expected parameter counts to avoid suggesting incompatible types.

• custom import message for case insensitive suggestion.
• only suggest case insensitive when no other suggestion is made
• check for enum variant match before suggesting an enum
• check for enum variant parameter count match before suggesting an enum
• squash and split commits for a clean PR

[GTimothy]

I see from the 247 failed tests that 1-edit difference is too big for small names. 'net' should probably NOT be suggested to replace 'new'...

Edit: i have now switched to case insensitive match only.