Buck: Fix infinite loop when trying to build non-existing core plugins

Running `buck build plugins:core` before initialising the submodules
causes buck to throw FileNotFoundException and then go into an infinite
loop.

The genrule in the plugins' BUCK configuration has a check in the `cmd`
to make sure the required core plugins (`NEED`) exist, but it looks
like this gets evaluated after the `srcs` which is using `CORE`.

Change `srcs` to use `HAVE`.  Now buck will exit with an error when the
core plugins do not exist.

Bug: issue 2343
Change-Id: I1bc2f67bcf6aacfe553c00e54fff18f959ac4b79
1 file changed