Define MAVEN_LOCAL for unpublished Maven JARs

Standardize the definition of MAVEN_LOCAL as ~/.m2/repository.
This makes it slightly easier to point to a custom build of a
dependency such as gwtorm or PrologCafe by installing the JAR to
the local Maven repository and updating the maven_jar() rule to
use repository = MAVEN_LOCAL instead of GERRIT.

Change-Id: I2e54d1039608c0195a992dbc12fe74bb513466c6

lib/maven.defs

@@ -14,6 +14,7 @@
 
 GERRIT = 'GERRIT:'
 MAVEN_CENTRAL = 'MAVEN_CENTRAL:'
+MAVEN_LOCAL = 'MAVEN_LOCAL:'
 
 def define_license(name):
   n = 'LICENSE-' + name

tools/download_file.py

@@ -26,6 +26,7 @@ from zipfile import ZipFile, BadZipfile, LargeZipFile
 REPO_ROOTS = {
   'GERRIT': 'http://gerrit-maven.storage.googleapis.com',
   'MAVEN_CENTRAL': 'http://repo1.maven.org/maven2',
+  'MAVEN_LOCAL': path.expanduser('~/.m2/repository'),
 }
 
 GERRIT_HOME = path.expanduser('~/.gerritcodereview')