Fix an import-related bug introduced by the `omitIndexFiles` flag

[cmaster11]

The bug would cause imports of generated types to point to the root generation directory (still expecting an index.ts file as if omitIndexFiles was false) instead of pointing to each individually generated file.

Given the custom scheme (in ./Inner.ts)

import Joi from 'joi';
export const InnerSchema = Joi.object({
  hello: Joi.string()
}).meta({ className: 'InnerInterface' });

Imported with (in ./FooBar.ts):

import Joi from 'joi';
import { InnerSchema } from './InnerSchema';

export const BarSchema = Joi.object({
  id: Joi.number().required().description('Id').example(1),
  inner: InnerSchema
}).meta({ className: 'Bar' });

Generates (in interfaces/FooBar.ts):

With bug (causing TS compilation issues):

import { InnerInterface } from '.'

After bug fix:

import { InnerInterface } from './Inner'

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (28fb60e) 98.62% compared to head (16e2a27) 98.63%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #353 +/- ## ======================================= Coverage 98.62% 98.63% ======================================= Files 9 9 Lines 508 511 +3 Branches 194 197 +3 ======================================= + Hits 501 504 +3 Misses 7 7 ``` | [Files Changed](https://app.codecov.io/gh/mrjono1/joi-to-typescript/pull/353?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jono) | Coverage Δ | | |---|---|---| | [src/writeInterfaceFile.ts](https://app.codecov.io/gh/mrjono1/joi-to-typescript/pull/353?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jono#diff-c3JjL3dyaXRlSW50ZXJmYWNlRmlsZS50cw==) | `100.00% <100.00%> (ø)` | |

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