Reflect variant name in directory ids

[mhegazy]

This is a follow up change to #428. In this PR we are making directory IDs reflect the variant in the name of the install directory (e.g. Swift\Toolchains\6.2.0+Asserts\usr).

There is no functional change in this change, it is just directory ids. Directory names and which components are installed to which directories should not be changed.

This should make it easier to add new variants by following the same pattern, and will allow us to parametrize authoring that is shared between variants.

Naming convention:

• ToolchainsVersioned became ToolchainsVersionedAsserts -- this is our top-level toolchain install directory like Swift\Toolchains\0.0.0+Asserts
• Subdirectories like _usr_share became toolchains_asserts_usr_share to reflect the full path at install time i.e.``Swift\Toolchains\0.0.0+Asserts\usr\share`

[compnerd]

Can you do a s/toolchains_assert/toolchain_assert/? The use is for a single toolchain installation right?

[mhegazy]

Can you do a s/toolchains_assert/toolchain_assert/? The use is for a single toolchain installation right?

changed to use singular toolchain