Build blame cache as own java_library

This is a first step towards properly packaging of blame cache in Gerrit
server which is needed for [1]. [1] currently (PS-44) depends on the
whole gitiles-servlet and this leads to a:

  java.lang.NoClassDefFoundError: com/google/template/soy/SoyFileSet

caused by packaging the gitiles-servlet in the Gerrit core but without
all of its dependencies.

[1] https://gerrit-review.googlesource.com/72798

Change-Id: I9326b30962c21082b3d152eb6397ddcf767d1ce3

.buckconfig

diff --git a/.buckconfig b/.buckconfig
index adac013..7ab2a74 100644
--- a/.buckconfig
+++ b/.buckconfig
@@ -1,5 +1,6 @@
 [alias]
   all = //:all
+  blame-cache = //blame-cache:lib
   in = //:install
   mvn_install = //:install
   mvn_deploy = //:deploy