[Build] Don't resign already signed jars

[HannesWell]

Reduce the number of actual signing operations by not re-signing artifacts (in the I-build) are already signed, usually because they have been 'baseline-replaced'.

In order to ensure that signatures of baseline-replaced artifacts can be considered, the p2-baseline-replacement is moved before the eclipse-jarsigner-plugin.

Furthermore set 'defaultP2Metadata' to false to skip the default baseline replace.

Fixes #2134

This is a follow-up on #2140.

[HannesWell]

In order to ensure that signatures of baseline-replaced artifacts can be considered, the p2-baseline-replacement is moved before the eclipse-jarsigner-plugin.

Furthermore set 'defaultP2Metadata' to false to skip the default baseline replace.

I have analyzed and thought through this again and concluded that all we should change is to set resigningStrategy=DO_NOT_RESIGN. The subsequent invocation of the p2-metadata goal is necessary in order to update the metadata like size if the jar was altered through the signing process. Because we have the p2-metadata-default activated the baseline replacement that restores previous signings already happens (the first time) earlier.

Because in multiple sub-modules the source feature generation, which requires yet another p2-metadata invocation (usually named attach-p2-metadata) is, is not guarded in a profile only activated for features, there are in total three p2-metadata invocations for many eclipse-plugin-projects.
But that's another issue and ideally we would replace source-features by pure metadata, but that's again another issue.

To make a long story short I think this should be fine now as it is.
@akurtakov do you agree?

[akurtakov]

Yes, no resigning if signed should be fine.

[HannesWell]

Build failure is unrelated and should hopefully be fixed with eclipse-equinox/p2#528.