Support any license key in `build_spdx_license_expression`

```
>>> from licensedcode.cache import build_spdx_license_expression
>>> build_spdx_license_expression("aaaa")
Traceback (most recent call last):
File "", line 1, in
File "python3.10/site-packages/licensedcode/cache.py", line 513, in build_spdx_license_expression
validate_spdx_license_keys(license_expression=license_expression, licensing=licensing)
File "python3.10/site-packages/licensedcode/cache.py", line 552, in validate_spdx_license_keys
raise InvalidLicenseKeyError(messages)
licensedcode.cache.InvalidLicenseKeyError: ['License key: aaaa is not a valid license key from LicenseDB', 'License key: aaaa is not a valid license key from LicenseDB']
```