Parse purls cautiously in getDeniedChanges

[juxtin]

Closes #752

Bottom line: the problems in the linked issue appear to be caused by the fact that we've been playing fast and loose with package-urls in a few different ways:

1. For deny-groups, we try to accept package-urls with a namespace but no name, but those kinds of package-urls are invalid according to the spec. In order to get around that, we sort of abuse the packageurl-js library.
2. While comparing actual dependencies against denied dependencies, we try to use the package_url member from the comparison API. However, it appears that this isn't always populated. Specifically, when a dependency has a version range like ^1.2.3, there's no way to format it into a package-url, so we just don't. These missing package_urls have been causing runtime exceptions.
3. While it's true that the purl-spec requires the namespace, name, and version fields to be percent-encoded, most implementations will tolerate a purl like pkg:npm/@ns/pkg@1.0.0 even though it's technically invalid. This is helpful for our users, many of whom probably don't know the minutiae of the package-url spec. The packageurl-js library does not make this affordance.

What I've done in this PR is write a very simple, permissive parser for package-urls that fits our purposes more closely than packageurl-js does. It allows us to parse partial purls, reports errors without throwing them, and tolerates special characters as long as the whole purl can still be unambiguously parsed.

In addition, I've made the following changes/improvements:

1. Standardized on a format for deny-groups that requires a namespace to be present. We used to tell users to use a purl fragment like pkg:npm/express to block the express namespace, but pkg:npm/express is a complete and valid purl that specifies the express package. Instead, the format should be pkg:npm/express/, where the trailing/ indicates that express is a namespace and not a package name.
2. Better config validation up front, so that users will know if they've included a package-url that we can't actually use. Validation for deny-groups requires a namespace.
3. If there's no package_url for a dependency, get the namespace from the package name.

In my test repositories reproducing the error reports we've gotten from users, these changes have successfully fixed the problems.

[bteng22]

Just a few clarifying comments, but overall LGTM 👍 Thanks for the help on this one and sorry for the release headaches. It should be have been a smell when we had to hack packageurl-js to fit our use case.

The build is breaking, but it looks like it's because it's pulling from the main branch and still using the packageurl-js so we should be good to merge here?

[bteng22]

👍

[juxtin]

The build is breaking, but it looks like it's because it's pulling from the main branch and still using the packageurl-js so we should be good to merge here?

That's right, yeah.