fix: move SSDK source code to 'src' folder

[trivikr]

Issue #, if available:
Fixes server protocol tests.
Example failure: https://github.com/aws/aws-sdk-js-v3/runs/3747465220

Description of changes:
Move SSDK source code to 'src' folder

Testing:

Verified that source files are generated in "src" folder and build is successful.

The tests fails with the following error, but it's unrelated to moving source code to 'src' folder in protocol_tests/aws-restjson-server

lerna ERR! yarn run build exited 2 in '@aws-sdk/aws-restjson-server'
lerna ERR! yarn run build stdout:
$ yarn build:cjs && yarn build:es && yarn build:types
$ tsc -p tsconfig.json
src/protocols/Aws_restJson1.ts(38,3): error TS2305: Module '"@aws-smithy/server-common"' has no exported member 'acceptMatches'.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

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