Remove processing of unused plugin element attributes in feature.xml and distinction between Plug-ins and Fragments in ProductFile

[HannesWell]

This PR consists of two parts:

1. Remove unused distinction between Plug-ins and Fragments in ProductFile
There is no usage of a product's fragments in P2 or PDE.
Consequently PDE support for adding a fragment attribute for fragments listed in a product file can be removed too.

Remove processing of unused plugin element attributes in feature.xml
Which happens in the context of eclipse-pde/eclipse.pde#730
From the removed attributes only the unpack attribute has been considered in in the FeaturesAction and the RemoteFeaturesAction in order to create corresponding BundleShapeAdvice.
But at least for Tycho those advies did not have any effect.

I assume, but I haven't verified it, that BundleShapeAdvices might have an effect in PDE-build tasks. But since a bundle itself should specify its bundle-shape in its MANIFEST.MF I'm fine with removing that support from PDE with this.
Nevertheless it requires some rework of the PDE-builds task since they use FeatureEntry.isUnpack()/unpackSet()/..., therefore this is currently a draft.

[github-actions]

Test Results

       6 files   -     3         6 suites   - 3   34m 12s ⏱️ + 1m 45s
2 065 tests  - 116  2 063 ✔️  - 114  1 💤  - 3  1 ❌ +1 
6 285 runs   - 348  6 280 ✔️  - 342  2 💤  - 9  3 ❌ +3 

For more details on these failures, see this check.

Results for commit 1d56fea. ± Comparison against base commit d78c767.

This pull request removes 116 tests.

org.eclipse.equinox.p2.tests.importexport.ImportExportRemoteTests ‑ Unknown test
org.eclipse.equinox.p2.tests.importexport.ImportExportTests ‑ testAllowExportFeaturesInstalledFromLocal
org.eclipse.equinox.p2.tests.importexport.ImportExportTests ‑ testExportFeaturesInstalledFromLocal
org.eclipse.equinox.p2.tests.importexport.ImportExportTests ‑ testIncompatibleP2f
org.eclipse.equinox.p2.tests.importexport.ImportExportTests ‑ testLoadP2f
org.eclipse.equinox.p2.tests.importexport.ImportExportTests ‑ testLoadUnknownP2f
org.eclipse.equinox.p2.tests.ui.actions.ElementUtilsTest ‑ testElements
org.eclipse.equinox.p2.tests.ui.actions.ElementUtilsTest ‑ testEmpty
org.eclipse.equinox.p2.tests.ui.actions.ElementUtilsTest ‑ testIUs
org.eclipse.equinox.p2.tests.ui.actions.ElementUtilsTest ‑ testInvalid
…

♻️ This comment has been updated with latest results.

[laeubi]

@HannesWell is this still relevant? It requires a manual rebase now!