Allow spaces in path requirements

[charliermarsh]

Summary

This PR modifies our parsing to allow spaces in URLs. I don't know if this is a correct change... But we now parse URLs until we see:

• A newline.
• A semicolon (marker) or hash (comment), preceded by a space. We parse the URL until the last
  non-whitespace character (inclusive).
• A semicolon (marker) or hash (comment) followed by a space. We treat this as an error, since
  the end of the URL is ambiguous (e.g., https://foo.com; marker) would be a URL that ends in ;).

Closes #6032.