Invalid and incomplete PackageUrl in ConanComponent

[RodneyRichardson]

ConanComponent.PackageUrl incorrectly specifies String.Empty for namespace and subpath (instead of null), resulting in something like pkg:conan//openssl@3.4.1# instead of pkg:conan/openssl@3.4.1. Compare with cargo fix in #1410.

It also does not support all specification options from https://github.com/package-url/purl-spec/blob/main/PURL-TYPES.rst#conan, for example:

pkg:conan/openssl.org/openssl@3.0.3?user=bincrafters&channel=stable
pkg:conan/openssl.org/openssl@3.0.3?arch=x86_64&build_type=Debug&compiler=Visual%20Studio&compiler.runtime=MDd&compiler.version=16&os=Windows&shared=True&rrev=93a82349c31917d2d674d22065c7a9ef9f380c8e&prev=b429db8a0e324114c25ec387bfd8281f330d7c5c

This might be practical to fix at the same time as adding Conan 2.0 support (#1228).