modules: Switch from declare to readonly, export

[mbland]

Encountered in the course of implementing #36. While writing the upcoming @go.log_add_output_file function in thelog module, which includes a call to . "$_GO_USE_MODULES" 'file', a test that made multiple calls to this new function would fail with the message:

 ✗ log/add-output-file: add output files for a mix of levels
   (in test file tests/log/add-output-file.bats, line 83)
     `"@go.log_add_output_file '$TEST_GO_ROOTDIR/info.log' 'INFO'" \' failed
   line 0 not equal to expected value:
     expected: 'INFO   FYI'
     actual:   'No file descriptors <  available; failed at:'

This error message is part of @go.open_file_or_duplicate_fd, and includes a reference to $_GO_MAX_FILE_DESCRIPTORS, which was previously declared as:

  declare -r _GO_MAX_FILE_DESCRIPTORS="${_GO_MAX_FILE_DESCRIPTORS:-256}"

The fact that no value appeared for "$_GO_MAX_FILE_DESCRIPTORS" was suspicous. Moving the . "$_GO_USE_MODULES" 'file' call outside of the function to the top level of the module got the test to pass; I understood this to be because:

• Top-level declare statements cause variables to be scoped to the
  function sourcing the file.
• When the function call ends, the declared variables go out of scope.
• Since "$_GO_USE_MODULES" ensures module files are imported only once,
  these variables aren't redeclared for subsequent calls.

After deciding that it should be permissible for a function to call . "$_GO_USE_MODULES" and have it work across multiple calls, I eventually learned that substituing readonly for declare -r did what I originally intended. The same also holds for export vs declare or declare -x. Making these updates to the file module allowed the test to pass; only minor changes were required to other tests when applying the updates to every module.

For now, this update applies only to module code. I may look into how to apply readonly and export to other code as well, but an initial experiment with replacing values in go-core.bash caused ./go test vars to fail.

[coveralls]

Coverage remained the same at 94.435% when pulling fae51c6 on readonly into b61556b on master.