eliminate app.builds everywhere, it should be app['Builds']

The .txt format was the last place where the lowercase "builds" was used,
this converts references everywhere to be "Builds".  This makes it possible
to load metadata YAML files with any YAML parser, then have it possible to
use fdroidserver methods on that data, like metadata.write_metadata().

The test files in tests/metadata/dump/*.yaml were manually edited by cutting
the builds: block and putting it the sort order for Builds: so the contents
should be unchanged.

```
sed -i \
 -e 's/app\.builds/app.get('Builds', \[\])/g' \
 -e "s/app\.get(Builds, \[\]) =/app\['Builds'] =/g" \
 -e "s/app\.get(Builds, \[\]) =/app\['Builds'] =/g" \
 -e "s/app\.get(Builds, \[\])/app.get('Builds', \[\])/g" \
 -e "s/app\.get('Builds', \[\])\.append/app\['Builds'\].append/g" \
 -e "s/app\['builds'\]/app.get('Builds', [])/g" \
 */*.*
```

fdroidserver/metadata.py

@@ -158,7 +158,7 @@ class App(dict):
 
         self.id = None
         self.metadatapath = None
-        self.builds = []
+        self.Builds = []
         self.comments = {}
         self.added = None
         self.lastUpdated = None
@@ -179,8 +179,8 @@ class App(dict):
             raise AttributeError("No such attribute: " + name)
 
     def get_last_build(self):
-        if len(self.builds) > 0:
-            return self.builds[-1]
+        if len(self.Builds) > 0:
+            return self.Builds[-1]
         else:
             return Build()
 
@@ -633,9 +633,6 @@ def post_metadata_parse(app):
         if type(v) in (float, int):
             app[k] = str(v)
 
-    if 'Builds' in app:
-        app['builds'] = app.pop('Builds')
-
     if 'flavours' in app and app['flavours'] == [True]:
         app['flavours'] = 'yes'
 
@@ -664,8 +661,8 @@ def post_metadata_parse(app):
     _bool_allowed = ('maven', 'buildozer')
 
     builds = []
-    if 'builds' in app:
-        for build in app['builds']:
+    if 'Builds' in app:
+        for build in app.get('Builds', []):
             if not isinstance(build, Build):
                 build = Build(build)
             for k, v in build.items():
@@ -693,7 +690,7 @@ def post_metadata_parse(app):
                                 build[k] = str(v)
             builds.append(build)
 
-    app.builds = sorted_builds(builds)
+    app['Builds'] = sorted_builds(builds)
 
 
 # Parse metadata for a single application.
@@ -710,8 +707,6 @@ def post_metadata_parse(app):
 #
 # Known keys not originating from the metadata are:
 #
-#  'builds'           - a list of dictionaries containing build information
-#                       for each defined build
 #  'comments'         - a list of comments from the metadata file. Each is
 #                       a list of the form [field, comment] where field is
 #                       the name of the field it preceded in the metadata
@@ -771,8 +766,8 @@ def parse_metadata(metadatapath, check_vcs=False, refresh=True):
     post_metadata_parse(app)
 
     if not app.id:
-        if app.builds:
-            build = app.builds[-1]
+        if app.get('Builds'):
+            build = app['Builds'][-1]
             if build.subdir:
                 root_dir = build.subdir
             else:
@@ -925,9 +920,8 @@ def write_yaml(mf, app):
                 insert_newline = True
             else:
                 if app.get(field) or field == 'Builds':
-                    # .txt called it 'builds' internally, everywhere else its 'Builds'
                     if field == 'Builds':
-                        if app.get('builds'):
+                        if app.get('Builds'):
                             cm.update({field: _builds_to_yaml(app)})
                     elif field == 'CurrentVersionCode':
                         cm.update({field: _field_to_yaml(TYPE_INT, getattr(app, field))})
@@ -945,7 +939,7 @@ def write_yaml(mf, app):
 
     def _builds_to_yaml(app):
         builds = ruamel.yaml.comments.CommentedSeq()
-        for build in app.builds:
+        for build in app.get('Builds', []):
             b = ruamel.yaml.comments.CommentedMap()
             for field in build_flags:
                 value = getattr(build, field)