tt CLI (2.7): use TT_CLI_MODULES_PATH variable to find modules

Product: tt cli
Since: tt 2.7.0
Root document: ?
SME: @ oleg-jukovec @ dmyger

Details

The TT_CLI_MODULES_PATH environment variable specifies a list of
module's paths separated by a colon “:”.
It can be set externally from tt and must contain a list of
folders where modules are located.
The logic of working with this variable is similar to the PATH
system variable.

Lists of modules are combined into one common list:

• The modules declared in the configuration file tt.yaml come first
• then the modules defined via the environment variable are added.
  Any duplicates must be removed, while maintaining the original
  order of module declarations.

Usage example
Set variable:

export TT_CLI_MODULES_PATH=/ext/path/modules:${TT_CLI_MODULES_PATH}

Requested by @dmyger in https://github.com/tarantool/tt/commit/877f3afb136b84b39f86f90df94f43e50b0744be.

Start by locating the tt CLI documentation and the section describing module discovery, using TT_CLI_MODULES_PATH and tt.yaml as search terms. Document the colon-separated environment variable, its ordering and duplicate-removal rules, and the usage example; done means the tt 2.7 behavior is explained clearly.