deprecate tendermint/spm  #1549

and move its content under starport/pkg/.

the initial idea with tendermint/spm was to have a lean repo to keep collection of dependencies that can be used by starport scaffold chains.

we wanted to separate these packages from starport's repo because go geting starport takes lots of time due to commit history. but this is not always true. when you go get an untagged version of starport, yes it git clones (without depth 1) the whole repo. but if you just go get the latest tagged (release) version which is the default behaviour of go get, it's only 122MB and downloaded as a single file (e.g. https://proxy.golang.org/github.com/tendermint/starport/@v/v0.17.2.zip) through Go proxy this time.

so, I propose to move spm back to make things easier to maintain.

after deprecating, we can safely delete the tendermint/spm repo. this won't break existent chains because Go proxy will never delete a tagged and cached package.