requirement: Improve error message for unpinned URL requirements

[tetsuo-cpp]

Closes #354

[tetsuo-cpp]

(env) tetsuo@Alexs-MacBook-Pro pip-audit % pip-audit -r requirements.txt --no-deps
WARNING:pip_audit._cli:--no-deps is supported, but users are encouraged to fully hash their pinned dependencies
WARNING:pip_audit._cli:Consider using a tool like `pip-compile`: https://pip-tools.readthedocs.io/en/latest/#using-hashes
Traceback (most recent call last):
  File "/Users/tetsuo/Code/pip-audit/env/bin/pip-audit", line 8, in <module>
    sys.exit(audit())
  File "/Users/tetsuo/Code/pip-audit/pip_audit/_cli.py", line 432, in audit
    for (spec, vulns) in auditor.audit(source):
  File "/Users/tetsuo/Code/pip-audit/pip_audit/_audit.py", line 66, in audit
    for dep, vulns in self._service.query_all(specs):
  File "/Users/tetsuo/Code/pip-audit/pip_audit/_service/interface.py", line 150, in query_all
    for spec in specs:
  File "/Users/tetsuo/Code/pip-audit/pip_audit/_dependency_source/requirement.py", line 120, in collect
    for _, dep in self._collect_cached_deps(filename, reqs):
  File "/Users/tetsuo/Code/pip-audit/pip_audit/_dependency_source/requirement.py", line 318, in _collect_cached_deps
    for req, dep in self._collect_preresolved_deps(
  File "/Users/tetsuo/Code/pip-audit/pip_audit/_dependency_source/requirement.py", line 265, in _collect_preresolved_deps
    raise RequirementSourceError(
pip_audit._dependency_source.requirement.RequirementSourceError: requirement de-core-news-sm is not pinned, URL requirements must be pinned with #egg=your_package_name==your_package_version: de-core-news-sm from https://github.com/explosion/spacy-models/releases/download/de_core_news_sm-3.2.0/de_core_news_sm-3.2.0.tar.gz#egg=de-core-news-sm (from RequirementLine(line_number=1, line='https://github.com/explosion/spacy-models/releases/download/de_core_news_sm-3.2.0/de_core_news_sm-3.2.0.tar.gz#egg=de-core-news-sm', filename=PosixPath('requirements.txt')))

[woodruffw]

LGTM!