[code-reflection] RFR: Turn compiler plugin into simple SPI

[Maurizio Cimadamore]

This PR is a small followup to the changes to turn compiler code reflection support into a compiler plugin.
Turns out that the new plugin required several hacks and workaround, to make sure that the plugin would only run when strictly required (not to affect outcome of compilation if the plugin was disabled). We can get rid of all such workarounds by turning the plugin into a plain and simple SPI. There is a new `ReflectMethodsProxy` interface, which is implemented by the new module (see `ReflectMethods.Provider`). This allows javac to more or less call into the new pipeline step as it did before the incubating change, which leads to code that is much simpler to maintain.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/babylon/pull/275/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=275&range=00
  Stats: 289 lines in 8 files changed: 103 ins; 181 del; 5 mod
  Patch: https://git.openjdk.org/babylon/pull/275.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/275/head:pull/275

PR: https://git.openjdk.org/babylon/pull/275