Address packages without a name in order to generate a valid purl

[pombredanne]

Description

Some package manifest may be missing some esssential identifying info such as a name or version.
This could be the case when these are dynamically generated at build time or when the package type allows this (such as bower, npm or composer private packages) . We should still capture this and not balk as there can be a lot of useful data beyond this

[steven-esser]

@pombredanne Did we determine that packages without a name should NOT be allowed?

[pombredanne]

@MaJuRG I am split on this. For now they are allowed. BUT they are super rare.

On the one hand, once a package has no name, its manifest becomes close to a mere build_script than a proper package_manifest as discussed in #1237.

On the other hand they may still contain valuable license and origin data.

So I may be inclined to allow an empty name and to add an explicit private flag attribute when a package has no name?