[skara] RFR: build: use included build instead of buildSrc

[Erik Helin]

Hi all,

please review this small patch that makes the build use "included builds" for Skara's Gradle plugins instead of the "buildSrc" feature of Gradle. The "buildSrc" feature doesn't play nice with Gradle's caching mechanism - any change to a file in the `buildSrc` directory invalidates the build cache. The solution is to just include the plugins using the `includeBuild` feature (the Gradle team is even thinking of making `buildSrc` an implicitly included build, see https://github.com/gradle/gradle/issues/2531).

Testing:
- [x] `make images` on Linux x64
- [x] `make test` on Linux x64

Thanks,
Erik

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Commit messages:
 - build: use included build instead of buildSrc

Changes: https://git.openjdk.java.net/skara/pull/1004/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1004&range=00
  Stats: 71 lines in 28 files changed: 32 ins; 27 del; 12 mod
  Patch: https://git.openjdk.java.net/skara/pull/1004.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1004/head:pull/1004

PR: https://git.openjdk.java.net/skara/pull/1004