Add --envrc-dir flag to allow specifying location of direnv config

[pstephan-geico-external]

Summary

Fixes #2459 - devbox generate direnv does not respect --config

It seems that the previous mode of operation for devbox generate direnv --config <some-dir> used --config to determine the location of the resulting .envrc file, rather than as the location of the devbox.json file. But in some cases it is desired to be able to specify not only the location of not only the direnv .envrc file, but also the devbox config file. Or at least to be able to specify just the location of the devbox config with the .envrc being in the current directory.

To accomplish this, without breaking the current mode of operation, a new parameter is added, --envrc-dir, which specifies the location of the resulting .envrc file. For example, the command devbox generate direnv --envrc-dir ABC --config ABC/XYZ would create ABC/.envrc and within that would be the command eval "$(devbox generate direnv --print-envrc --config XYZ)"

Without the new --envrc-dir param, operation is the same as it was previously, even with the --config (which will be used to specify the location of the .envrc file, NOT the devbox config file, devbox.json.

How was it tested?

Several new tests have been created to cover cases with and without the new --envrc-dir param.

Community Contribution License

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maintainers under the terms of the
Apache 2 License.

By creating this pull request, I represent that I have the right to license the
contributions to the project maintainers under the Apache 2 License as stated in
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Community Contribution License.

[darch-geico-external]

I think I only spotted one actual bug.

[darch-geico-external]

Obviously still needs review from an actual maintainer, but I don't see any other issues from here.