Option to run from the source directory without .terragrunt-cache

Describe the enhancement

Add an option to run Terraform/OpenTofu from the source unit directory without using .terragrunt-cache.

In v0.99.x, Terragrunt ran directly from the unit directory when no terraform { source } block was set. In v1, it always runs from within .terragrunt-cache, which breaks our workflow in two ways:

1. Relative local module paths stop resolving — paths that correctly pointed to local modules when running from the unit directory no longer resolve correctly when running from inside .terragrunt-cache.

2. generate output is invisible to IDE tooling — in v0.99.x, generate wrote files (including variable declarations) to the unit directory, where IDE extensions could read them. In v1, those files are written to .terragrunt-cache, so IDE extensions cannot find the declarations and report errors.

Because of these issues, we are unable to upgrade from v0.99.x to v1.

Additional context

• Changes required: add an option to skip .terragrunt-cache and run directly from the source unit directory.
• Implications of the feature: units without terraform { source } would run in-place, preserving relative module paths and making generate output visible to IDE tooling.
• Alternatives considered: copying parent directories into the cache, using symlinks — both introduce maintenance burden and are not viable long-term solutions.
• Level of effort: unclear, but a simple flag that preserves v0.99.x behavior for units without terraform { source } may be a starting point.

PoC (Proof of Concept)

• PoC

RFC Not Needed

• I have evaluated the complexity of this enhancement, and I believe it does not require an RFC.