Move pom.xml files to tools/maven

In preparation for dissolving the various top-level directories that
contain these projects. This was previously attempted in 0ce2fdc1, but
that commit included a change to how the pom.xml files are generated
which was not compatible with Nexus's requirements.

This attempt does the minimal work necessary, and simply moves the
pom.xml files to a new location under tools, pointing our existing
deployment scripts at the new location.

Tested:
* Ran `./tools/maven/api.sh deploy` and sanity checked the resulting
  published snapshot pom.xml:
  https://oss.sonatype.org/content/repositories/snapshots/com/google/gerrit/gerrit-extension-api/2.16-SNAPSHOT/gerrit-extension-api-2.16-20171010.133016-3.pom
* Ran './tools/version.py 2.3456` and checked that all *_pom.xml files
  were modified.

Change-Id: I24e5e3bd8d24dddc44192fb86c3905a79d7c296c

Documentation/dev-release.txt

@@ -79,7 +79,7 @@ the `gerrit-security-fixes` project be taken over into the public
 Before doing the release build, the `GERRIT_VERSION` in the `version.bzl`
 file must be updated, e.g. change it from `$version-SNAPSHOT` to `$version`.
 
-In addition the version must be updated in a number of pom.xml files.
+In addition the version must be updated in a number of `*_pom.xml` files.
 
 To do this run the `./tools/version.py` script and provide the new
 version as parameter, e.g.:
@@ -137,7 +137,7 @@ link:dev-release-deploy-config.html#deploy-configuration-setting-maven-central[
 configuration] for deploying to Maven Central
 
 * Make sure that the version is updated in the `version.bzl` file and in
-the `pom.xml` files as described in the link:#update-versions[Update
+the `*_pom.xml` files as described in the link:#update-versions[Update
 Versions and Create Release Tag] section.
 
 * Push the WAR to Maven Central:
@@ -192,7 +192,7 @@ section, e.g. https://oss.sonatype.org/content/repositories/comgooglegerrit-1029
 +
 Use this URL for further testing of the artifacts in this repository,
 e.g. to try building a plugin against the plugin API in this repository
-update the version in the `pom.xml` and configure the repository:
+update the version in the `*_pom.xml` and configure the repository:
 +
 ----
   <repositories>

tools/maven/mvn.py

@@ -56,7 +56,7 @@ else:
 for spec in args.s:
   artifact, packaging_type, src = spec.split(':')
   exe = cmd + [
-    '-DpomFile=%s' % path.join(root, '%s/pom.xml' % artifact),
+    '-DpomFile=%s' % path.join(root, 'tools', 'maven', '%s_pom.xml' % artifact),
     '-Dpackaging=%s' % packaging_type,
     '-Dfile=%s' % src,
   ]

tools/version.py

@@ -48,7 +48,7 @@ src_pattern = re.compile(r'^(\s*<version>)([-.\w]+)(</version>\s*)$',
 for project in ['gerrit-acceptance-framework', 'gerrit-extension-api',
                 'gerrit-plugin-api', 'gerrit-plugin-gwtui',
                 'gerrit-war']:
-  pom = os.path.join(project, 'pom.xml')
+  pom = os.path.join('tools', 'maven', '%s_pom.xml' % project)
   replace_in_file(pom, src_pattern)
 
 src_pattern = re.compile(r'^(GERRIT_VERSION = ")([-.\w]+)(")$', re.MULTILINE)