Cross-build to 2.13

[bjaglin]

sbt-scalafix users will soon be able to use _2.13 rules (although the default will remain _2.12 for a while), see scalacenter/scalafix#1108

[bjaglin]

I left this here as I fixed the deprecation warnings

[codecov]

Codecov Report

Merging #69 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #69   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           10        10           
=========================================
  Hits            10        10           

Impacted Files	Coverage Δ
...les/src/main/scala/fix/OrganizeImportsConfig.scala	100.00% <100.00%> (ø)

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[bjaglin]

Coverage seems to be computed for 2.12 and 2.13 separately as expected (I guess we want that when/if code starts drifting?), although it's unclear how it's reported

==> Reading reports
    + ./rules/target/scala-2.13/coverage-report/cobertura.xml bytes=75862
    + ./rules/target/scala-2.13/scoverage-report/scoverage.xml bytes=197420
    + ./rules/target/scala-2.12/coverage-report/cobertura.xml bytes=83440
    + ./rules/target/scala-2.12/scoverage-report/scoverage.xml bytes=219528
==> Appending adjustments
    https://docs.codecov.io/docs/fixing-reports
    -> No adjustments found
==> Gzipping contents
==> Uploading reports
    url: https://codecov.io
    query: branch=213&commit=a42dbba18e8d08cc278ded3d99595da1ca8ff432&build=119139575&build_url=http%3A%2F%2Fgithub.com%2Fliancheng%2Fscalafix-organize-imports%2Factions%2Fruns%2F119139575&name=&tag=&slug=liancheng%2Fscalafix-organize-imports&service=github-actions&flags=&pr=69&job=
    -> Pinging Codecov
https://codecov.io/upload/v4?package=bash-20200430-d757c17&token=secret&branch=213&commit=a42dbba18e8d08cc278ded3d99595da1ca8ff432&build=119139575&build_url=http%3A%2F%2Fgithub.com%2Fliancheng%2Fscalafix-organize-imports%2Factions%2Fruns%2F119139575&name=&tag=&slug=liancheng%2Fscalafix-organize-imports&service=github-actions&flags=&pr=69&job=
    -> Uploading
    -> View reports at https://codecov.io/github/liancheng/scalafix-organize-imports/commit/a42dbba18e8d08cc278ded3d99595da1ca8ff432

[liancheng]

@bjaglin, this is awesome! I've been wanting this for quite a while but hesitated since I'm not that familiar with SBT, especially recent versions. Thanks for working on it!

[liancheng]

For Codecov report, it is actually broken from day one when I introduced it. Somehow the report does not cover the main implementation file OrganizeImports.scala. See #54.

[bjaglin]

@liancheng it's actually very recent, and for now incentive to cross-build is limited as it's not possible to use it via sbt-scalafix. I am actively working on this though, as I happen to have rules local to a 2.13 project running in the same invocation as OrganizeImports, so I having everything in 2.13 would make the build much simpler. Thus this PR!