Refactor ICU4X code to allow configured code by version

[sffc]

Unblocks #372

[sven-oly]

Problems encountered in using this in all versions (1.3, 1.4, 1.5, and 2.0_beta)*

$ gh pr checkout 377
remote: Enumerating objects: 122, done.
remote: Counting objects: 100% (122/122), done.
remote: Compressing objects: 100% (51/51), done.
remote: Total 122 (delta 67), reused 119 (delta 64), pack-reused 0 (from 0)
Receiving objects: 100% (122/122), 27.33 KiB | 6.83 MiB/s, done.
Resolving deltas: 100% (67/67), c$ gh pr checkout 377
remote: Enumerating objects: 122, done.
remote: Counting objects: 100% (122/122), done.
remote: Compressing objects: 100% (51/51), done.
remote: Total 122 (delta 67), reused 119 (delta 64), pack-reused 0 (from 0)
Receiving objects: 100% (122/122), 27.33 KiB | 6.83 MiB/s, done.
Resolving deltas: 100% (67/67), completed with 3 local objects.
From github.com:unicode-org/conformance

• [new branch] icu4x-refactor -> upstream/icu4x-refactor
  branch 'icu4x-refactor' set up to track 'upstream/icu4x-refactor'.
  Switched to a new branch 'icu4x-refactor'
  ccornelius@ccornelius3:/ICU_conformance/conformance/executors/node$ cd ..
  ccornelius@ccornelius3:/ICU_conformance/conformance/executors$ cd rust
  ccornelius@ccornelius3:/ICU_conformance/conformance/executors/rust$ cd 1.3
  ccornelius@ccornelius3:/ICU_conformance/conformance/executors/rust/1.3$ cargo build
  Compiling executor v0.0.0 (/usr/local/google/home/ccornelius/ICU_conformance/conformance/executors/rust/1.3)
  error[E0658]: using an imported function as entry point main is experimental
  --> build.rs:4:5
  |
  4 | use print_icu4x_versions_1_3::main;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #28937 Tracking issue for Allow a re-export for main (RFC 1260) rust-lang/rust#28937 for more information

For more information about this error, try rustc --explain E0658.
error: could not compile executor (build script) due to previous error
ccornelius@ccornelius3:~/ICU_conformance/conformance/executors/rust/1.3$ cargo build --release
ompleted with 3 local objects.
From github.com:unicode-org/conformance

• [new branch] icu4x-refactor -> upstream/icu4x-refactor
  branch 'icu4x-refactor' set up to track 'upstream/icu4x-refactor'.
  Switched to a new branch 'icu4x-refactor'
  ccornelius@ccornelius3:/ICU_conformance/conformance/executors/node$ cd ..
  ccornelius@ccornelius3:/ICU_conformance/conformance/executors$ cd rust
  ccornelius@ccornelius3:/ICU_conformance/conformance/executors/rust$ cd 1.3
  ccornelius@ccornelius3:/ICU_conformance/conformance/executors/rust/1.3$ cargo build
  Compiling executor v0.0.0 (/usr/local/google/home/ccornelius/ICU_conformance/conformance/executors/rust/1.3)
  error[E0658]: using an imported function as entry point main is experimental
  --> build.rs:4:5
  |
  4 | use print_icu4x_versions_1_3::main;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #28937 Tracking issue for Allow a re-export for main (RFC 1260) rust-lang/rust#28937 for more information

For more information about this error, try rustc --explain E0658.
error: could not compile executor (build script) due to previous error
ccornelius@ccornelius3:~/ICU_conformance/conformance/executors/rust/1.3$ cargo build --release

[sven-oly]

After rustup 1.80, this works fine.

[echeran]

LGTM

[echeran]

Interesting. I like the way you did this. I didn't know that you can configure which file Cargo should look at to find a particular module, but that's really convenient for this use case.