Specify target platform file in API Tools baseline

[lppedd]

Currently the API Tools mojo supports specifying baselines only via repository URLs.
However, a more convenient way would be to use an existing .target file, being it is already shared between the IDE and Maven/Tycho.

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Edit - minor enhancement:

tycho/tycho-apitools-plugin/src/main/java/org/eclipse/tycho/apitools/ApiFileGenerationMojo.java

Lines 93 to 95 in 997b9a0

	if (eclipseProject.isEmpty()
	|| !eclipseProject.get().hasNature("org.eclipse.pde.api.tools.apiAnalysisNature")) {
	return;

Printing out why it's skipped might be better, at least the user is aware there is a missing nature.