Disallow trailing slashes in imports

From https://github.com/ethereum/solidity/pull/10976#discussion_r578594268

## Abstract
Currently the compiler ignores any number of slashes at the end of the name of an imported module. For example this test case is perfectly valid:

```solidity
==== Source: a/b/c.sol ====
contract C {}
==== Source: a/b/d.sol ====
import "./c.sol/";
contract D is C {}
```

I think that this should result in a compilation error instead. Only `import "./c.sol";` should be accepted.

## Motivation
I see no legitimate need for these trailing slashes. I haven't seen this used in practice and if I did, I would probably be confused by it - Solidity does not currently allow importing whole directories but it looks like such an import.

It's also a source of bugs due to libraries not supporting all edge cases correctly (e.g. #10713). Disallowing this would free us from worrying about some of these edge cases.

## Specification
If the path passed to `import` ends with `/`, the compiler should issue an error saying that it's not possible to import a directory. For consistency, the compiler might also issue this error if it determines that an import not ending with a slash is pointing at a directory.

Alternatively, given that user might not understand that the compiler sees a file with a trailing slash as a directory, the message could just say that imported paths cannot end with a slash.

## Backwards Compatibility
This will break code that uses this style of import. Such code is easy to fix by removing the slash but it's still a breaking change.

Start by tracing the compiler's handling of imported paths, using the Solidity import example in the issue as a regression case. Verify that an import ending in `/` is rejected with the intended diagnostic, while the equivalent path without the trailing slash remains accepted.

Written by the indexing model from the issue text.