Add wording that a purl type SHOULD be locatable

[pombredanne]

We should spec if and which type CANNOT be located. A purl of a given type SHOULD be locatable. Unlocatable purl are mostly harmless and should be rarely.

We could add this wording at the top:

• https://github.com/package-url/purl-spec/blob/main/PURL-TYPES.rst
• and also in the spec we should define for each type if it is locatable or not.