Fix substring matching on cpan and mlflow type checks

[arpitjain099]

Both the cpan and the mlflow type checks are missing a trailing comma:

if ptype and ptype in ("cpan"):
if ptype and ptype in ("mlflow"):

("cpan") is just the string "cpan", not a one-element tuple, so in runs a substring test instead of an equality test. Any type that happens to be a substring of the literal takes the branch.

On current main:

>>> PackageURL(type="pan", namespace="Some-Namespace", name="x").to_string()
'pkg:pan/SOME-NAMESPACE/x'

The namespace gets uppercased because "pan" in "cpan" is true. Same for "c", "cpa", "an" and so on. The mlflow one is the same shape, and it returns early, so a name normalized under type "flow" goes down the mlflow path and gets lowercased when a repository_url databricks qualifier is set.

No currently registered purl type is a substring of either literal, so this isn't biting a spec type today. It is reachable right now though, since arbitrary types are accepted, and it silently corrupts the namespace or name rather than erroring.

Switched both to ==, which is what the pypi, hackage and pub checks a few lines down already use. I also dropped the now-redundant ptype and guard since == "cpan" is already false for None and "".

I grepped the rest of src/ for the same in ("...") single-string shape and these two are the only instances.

Tests: added two cases to tests/test_packageurl.py covering both. Reverting the source change and keeping the tests fails them:

>           assert normalize_namespace("Some-Namespace", ptype) == "Some-Namespace"
E           AssertionError: assert 'SOME-NAMESPACE' == 'Some-Namespace'

>           assert normalize_name("MyModel", databricks, ptype) == "MyModel"
E           AssertionError: assert 'mymodel' == 'MyModel'

2 failed, 47 deselected

With the fix, pytest tests is 987 passed. black --line-length 100, isort --profile black and mypy are clean.

One note on overlap: #188 adds PURL_TYPES and touches this file, but it only adds a constant near the top and doesn't change these two lines, so it shouldn't conflict.