Move legacy src/core_plugins to src/legacy/core_plugins.

[azasypkin]

In this PR I move src/core_plugins to src/legacy/core_plugins as per #12466 (comment).

First commit just git moves core plugins to a new location and the rest commits are to fix paths within plugins and common code (easier to review commit by commit).

Initially I was planning to move plugin by plugin, but now I feel like it's safer to move it in one go to not break references between plugins and common code.

[elasticmachine]

Pinging @elastic/kibana-platform

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[epixa]

nit: packages/kbn-i18n/README.md has an example using core_plugins that should be updated. It's non-critical though since it's just an example. I didn't see any other references to core_plugins that needed to be updated.

Do not squash+merge. Git diffing works pretty well with renamed files, but it can get tripped up with certain changes when a commit both renames a file and makes changes to it, which can cause unnecessary git conflicts on existing PRs. Either use the rebase+merge or merge+with+commit option instead.

[azasypkin]

Do not squash+merge. Git diffing works pretty well with renamed files, but it can get tripped up with certain changes when a commit both renames a file and makes changes to it, which can cause unnecessary git conflicts on existing PRs. Either use the rebase+merge or merge+with+commit option instead.

Good point, but these two options are marked as Not enabled for this repository for me, how do we deal with that?

[epixa]

I just enabled rebase merge

[azasypkin]

Cleanly rebased on the latest master (with fixed CI) and waiting for green CI here to rebase + merge via GitHub.

[azasypkin]

Previous CI run failed because of this (looks like intermittent CI connectivity issue):

12:07:08 	Suppressed: hudson.remoting.Channel$CallSiteStackTrace: Remote call to JNLP4-connect connection from kibana-ci-immutable-centos-1543833139078561052.c.elastic-ci-prod.internal/10.224.0.166:46270
12:07:08 		at hudson.remoting.Channel.attachCallSiteStackTrace(Channel.java:1741)
12:07:08 		at hudson.remoting.UserRequest$ExceptionResponse.retrieve(UserRequest.java:357)
12:07:08 		at hudson.remoting.Channel.call(Channel.java:955)
12:07:08 		at hudson.FilePath.act(FilePath.java:1161)
12:07:08 		at com.google.jenkins.plugins.storage.AbstractUpload.initiateUploadsAtWorkspace(AbstractUpload.java:378)
12:07:08 		at com.google.jenkins.plugins.storage.AbstractUpload.perform(AbstractUpload.java:166)
12:07:08 		at com.google.jenkins.plugins.storage.AbstractUpload.perform(AbstractUpload.java:133)
12:07:08 		at com.google.jenkins.plugins.storage.GoogleCloudStorageUploader.perform(GoogleCloudStorageUploader.java:110)
12:07:08 		at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
12:07:08 		at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:744)
12:07:08 		at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:690)
12:07:08 		at hudson.model.Build$BuildExecution.post2(Build.java:186)
12:07:08 		at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:635)
12:07:08 		at hudson.model.Run.execute(Run.java:1840)
12:07:08 		at hudson.matrix.MatrixRun.run(MatrixRun.java:153)
12:07:08 		at hudson.model.ResourceController.execute(ResourceController.java:97)
12:07:08 		at hudson.model.Executor.run(Executor.java:429)
12:07:08 Caused by: java.net.SocketTimeoutException: connect timed out

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[azasypkin]

6.x backport is currently blocked by backport of #22944

[azasypkin]

6.x/6.6: 584a777..b5fc9dc