move the Java runfiles library to @bazel_tools//tools/java/runfiles

[laszlocsomor]

Description of the problem / feature request:

Every language's runfiles library is under @bazel_tools//tools/<language>/runfiles, Java being the only exception with @bazel_tools//tools/runfiles:java-runfiles.

Feature requests: what underlying problem are you trying to solve with this feature?

Consistency with other languages.

What's the output of bazel info release?

  $ bazel info release
release 0.16.0

Any other information, logs, or outputs that you want to share?

#5802 is not a blocker, but an annoyance, because we can't just add a deprecation attribute to @bazel_tools//tools/runfiles:java-runfiles telling users which target to use instead.

[laszlocsomor]

Workaround for the time being (because of #5802):

• add a new java_library to src/tools/runfiles/java/com/google/devtools/build/runfiles/BUILD.tools called "runfiles-deprecated" with a deprecation attribute
• change the alias rule in tools/runfiles/BUILD.tools to point to "runfiles-deprecated"
• add a new java_library to tools/java/runfiles/BUILD.tools that uses the sources from //src/tools/runfiles/java/com/google/devtools/build/runfiles:java-srcs.

[laszlocsomor]

This workaround is not good enough: it warns against the alias rule, not against the rule that depends on the alias, i.e. Bazel will warn against @bazel_tools//tools/runfiles:java-runfiles, not against the rule that depends on @bazel_tools//tools/runfiles:java-runfiles (as we would like to).