Fix gcp-repo/metadata.p2.inf so that category is published in normal Maven builds

[briandealwis]

Our CT4E category doesn't appear in the gcp-repo p2 repository when built from Maven. It does appear in our release builds as we explicitly re-publish artifacts as part of the release.

This fixes our metadata.p2.inf to specify an exact range to pull in the category definition. It also changes some naming to distinguish between the category IU and the capabilities:

• Use $version$ to use the version of the .product: no need to change the .p2.inf file for releases
• Append .definition to the capability name, used to pull in the category IU from the product IU, to avoid confusion with the actual category IU. This capability is required as Tycho's assemble-repository plugin only traverses the modules actually defined in this project, and it doesn't track the additional IUs defined in the .p2.inf.
• Change the category capability to always be version 1.0.0
• Change the requirement to be an exact version

(assemble-repository's docs](https://www.eclipse.org/tycho/sitedocs/tycho-p2/tycho-p2-repository-plugin/assemble-repository-mojo.html) note that it only follows IUs that are included, and

Dependencies with a strict version range, i.e. a range which only matches exactly one version of an artifact, are also considered as inclusions.
And perhaps the [1.0.0,1.2.0] range is considered open-ended, vs the [1.0.0,1.0.0] which is an exact specification.

Identified as part of #2027

[chanseokoh]

Can you update the first step of the release instructions too?

https://github.com/GoogleCloudPlatform/google-cloud-eclipse/wiki/Cloud-Tools-for-Eclipse-Release-Instructions

[codecov-io]

Codecov Report

Merging #2043 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #2043   +/-   ##
=========================================
  Coverage     66.05%   66.05%           
  Complexity     1868     1868           
=========================================
  Files           322      322           
  Lines         12542    12542           
  Branches       1142     1142           
=========================================
  Hits           8284     8284           
  Misses         3833     3833           
  Partials        425      425

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