clean: remove backward-compat checks for old Rollup versions

[agilgur5]

Summary

Clean up some old backward-compat checks that are no longer necessary for the minimum version of Rollup that rpt2 requires

• Follow-up to fix: type-check included files missed by transform (type-only files) #345 (comment), where I realized several of these checks were legacy and no longer needed

Details

• we require Rollup >=1.26.3 in the peerDeps and the README and have for years now

  • (and Rollup is currently at 2.75.7, so this is a pretty low target)
  • as such, we can remove various checks that are legacy backward-compat remnants for old Rollup versions:
    • this.meta was added to options in 1.1.0: https://github.com/rollup/rollup/blob/master/CHANGELOG.md#110
    • this.addWatchFile was added at least in 1.0.0: https://github.com/rollup/rollup/blob/master/CHANGELOG.md#100
    • this.error and this.warn were added to transform in 0.41.0: https://github.com/rollup/rollup/blob/master/CHANGELOG.md#0410

• this simplifies some of the code a decent bit, RollupContext in particular

  • see also the usage of buildEnd in fix: type-check included files missed by transform (type-only files) #345

• modify tests to account for these changes; basically just simplify them

  • and remove the check against private internals of the class, as they're private
    • guess I forgot to remove this when I was refactoring the test code in test: add initial unit test suite #321?

Future Work

• More follow-ups to refactor: prefer native methods to lodash where possible #328 to remove more lodash usage
  • The rest of the _.isFunction usage can be replaced with a check for the opposite condition, i.e. typeof message === "string" ? message : message()
  • _.merge, _.isEqual, and _.compact would be the only ones leftover after

[ezolenko]

btw, there is no reason not to bump minimum rollup version if we need to use anything new, I expect most people either update whole build toolchain, or freeze the lot. Well, all sane people would anyway... :)

[agilgur5]

btw, there is no reason not to bump minimum rollup version if we need to use anything new

Yea I figure, given how old the current min is, but that also would technically be a breaking change, so also wouldn't want to do so without good reason. So far, I haven't encountered anything that needed new APIs though (which is surprising, because as a Rollup user, it seemed like I had to update quite a lot for Rollup 2, but now, as a plugin developer, I'm not seeing the same need to upgrade 🤷 )