Completes revert of PR google#335.

BREAKING CHANGE: This PR removes the support that was added
in PR google#335 for aquiring Blockly directly from a git:// URL.

This feature was useful insofar as it enabled merging changes
into blockly-samples that depend on changes in blockly that
have not yet been published (even as a beta)—and still have
tests pass.  For this to work seamlessly, however, the code
in webpack.config.js depended on a postinstall script that
was removed in PR google#1630.

When testing such PRs going forward use npm link for local
testing and wait for changes to blockly to be published
before merging the corresponding changes to samples—or wait
for blockly to become a monorepo so both changes can be made
in the same PR!

Note that this change is breaking only to the dev-scripts plugin
itself, and will not cause other plugins that use it as a dev
dependency to experience a breaking change.

The commit which removed support for git:// URLS by completing
the revert of PR google#335 removed the initialisation of packageJson
(from the package.json of the plugin being built) which turns
out to still be needed by a DefinePlugin call added later.

PR google#226 addedd a resolve.alias for blockly to webpack.config.js.
It is not entirely clear what the purpose of this was at the
time, but it has the effect of treating imports of submodules
(e.g. 'blockly/core') as if they were direct imports (e.g. of
'./node_modules/blockly/core.js'), causing webpack to ignore
the blockly package's package.json file.

This causes plugins to fail to build due to the introduction
of an exports stanza in that file (and other related changes)
in google/blockly#7822.

Fix bloat caused by some plugins depending on all of blockly
(instead of just blockly/core), resulting in webpack including
a copy of blockly in the bundled plugin becuase only the
subpackage entrypoints were listed inthe externals stanza of
webpack.config.js.

This will also avoid certain problems that might occur due to
apps using such bundled inadvertently containing two or more
different copies of Blockly.

Also fix the one plugin which did still have an unnecessary dependency
on blockly intead of blockly/core.

Currently webpack.conf.js is hard to understand.  Attempt to
improve readability by making some parts more DRY ("don't
repeat yourself") and others more DAMP ("descriptive and
 meaningful phrases").

Add bufferutils and utf-8-validate to IgnorePlugin config when
building tests.  These are optional dependencies of wd, which is
itself a dependency of jsdom.

Also refactor how plugins config is generated to improve readability.

There doesn't appear to be any reason not to include the '.ts'
in resolve.extensions even for pure-JS plugins, but it is
_necessary_ to include it in TS plugins.

Since the default value for resolve.extensions is
    ['.js', '.json', '.wasm']
set it to
    ['.ts', '.js', '.json', '.wasm']
which gives priority to TS, then JS, then the other default
extensions.

Also add a helpful comment explaining the purpose of
resolve.fallback.

The latter has never been an advertised entrypoint, and will
cease to be a valid entrypoint in v11 (see google/blockly#7822).
Fortunately the 'blockly' entrypoint behaves the same as
the 'blockly/node' entrypoint does in a node.js environment.

Have runSerializationTestSuite accept a second argument which is
the Blockly object to use for XML serialization/deserialization.

This works around an issue in blockly-samples that (following
the deletion of the alias for blockly in webpack.config.js)
causes tests using this function to fail due to having two copies
of jsdom loaded, where each will reject DOM objects created by
the other.  See
google#2229 (comment)
for more details.