Make moduleVisitor extendable by plugins

Nobody has claimed this yet.

Hello,

This is a feature request (kind of a continuation of #906),

Some modules require module paths for some calls (like the `jest.mock()` in Jest). As a developer, I would like `eslint-plugin-import` to apply its rules to paths to modules in other places different than `require()` or `import` calls so that I can quickly catch possible issues with such calls (i.e. the path in some call is pointing to an unresolved module).

Given this requirement is out of the scope of `eslint-plugin-import` (it shouldn't/can't keep track of paths used in calls other different modules), it can instead provide an interface for other modules to extend the scope of the default `moduleVisitor` (like the one for custom resolvers).

For instance, As a module author, I should be able to write an `eslint-import-visitor-jest` module that exports a function such as:
```JavaScript
function checkJestMocks(visitors, checkSourceValue) {
const currentCallExpression = visitors['CallExpression'];
visitors['CallExpression'] = function (call) {
if (currentCallExpression) currentCallExpression(call)
if (call.callee.type !== 'MemberExpression') return
if (call.callee.object.name !== 'jest') return
if (call.callee.property.name !== 'mock') return
if (call.arguments.length !== 1 || call.arguments.length !== 2) return
const modulePath = call.arguments[0]
if (modulePath.type !== 'Literal') return
if (typeof modulePath.value !== 'string') return

checkSourceValue(modulePath, call)
}
}
module.exports.visitor = checkJestMocks;
```
Then a developer can include my module and point `eslint-plugin-import` to use the exported visitor via a setting.

I would like to write this new feature if you consider and accept a PR for this.

Start by locating the moduleVisitor entry point and reading the existing custom-resolver extension path described in the issue. Define how external visitors would be supplied through settings, how they would receive checkSourceValue, and what the completed plugin interface should support; the issue does not name files or tests.

Written by the indexing model from the issue text.