Loading plugins by path is blocked in newer TS versions for security reasons. Convert the test
plugins to packages and install them locally so we can include them in the tsconfig by name.

Moved fake plugin packages and various typescript versions to a package.json in the workspace. This
is installed before running tests and means we can also include the main plugin by name

Newer versions of TS only allow plugins to be specified by package name. Dropped the tests and the
code for guessing when the plugin is included via a local path. This will make no difference to
anyone using the plugin normally via npm, and won't make any difference to properly set up
projects.

Since we cannot load the plugin by filepath in newer TS versions, and we can't alias a package from
a local path, it's hard to install the plugin with a different name and generate warnings in the
tests. Added it awkwardly as a new name todo this.

Moved the default TS dependency to the project root to work around issues with packages that have
peer deps being installed as local folders. Adjusted stored paths to compensate. Included TS as a
peer dep for the dummy plugins that need it.