Improve error messaging for constraints violations.

1) Break up dense lines with clearer pretty-printing.
2) When a violation happens because of a select(), mention
   both the target with the select (as before) *and*
   the dep that the select() chose.
3) Integrate this messaging into --target_environment violations,
   which currently provide no info about the root cause.

Examples:

-------------------------------------
select() + compatible_with violation:
-------------------------------------
Before:
  ERROR: /workspace/testapp/BUILD:41:1: in cc_binary rule //testapp:top: the current command-line flags disqualify all supported environments because of incompatible select() paths:
 environment: //constraints:p removed by: //testapp:midlib (/workspace/testapp/BUILD:28:1)

After:
  ERROR: /workspace/testapp/BUILD:41:1: in cc_binary rule //testapp:top: the current command line flags disqualify all supported environments because of incompatible select() paths:

    environment: //constraints:p
      removed by: //testapp:midlib (/workspace/testapp/BUILD:28:1)
      which has a select() that chooses dep: //testapp:glib
      which lacks: //constraints:p.

-------------------------------------
select() + --target_environment=//constraints:p violation:
-------------------------------------
Before:
  ERROR: This is a restricted-environment build.
   - //testapp:top does not support required environment //constraints:p

After:
  ERROR: This is a restricted-environment build.

  //testapp:top does not support:
    environment: //constraints:p
      removed by: //testapp:midlib (/workspace/testapp/BUILD:28:1)
      which has a select() that chooses dep: //testapp:g
      which lacks: //constraints:p

Fixes: #5795
PiperOrigin-RevId: 207910308

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