make document loaders a standalone crate in repo

**Is your feature request related to a problem? Please describe.**
A clear and concise description of what the problem is. Ex. I'm always frustrated when I want to use document loaders for a simple task but I have to import the entire langchain crate

**Describe the solution you'd like**
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I would love to import document_loaders from the langchain repo directly in my toml file.

**Describe alternatives you've considered**
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**Additional context**
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