[fidlc] Sharing error formatting in ErrorReporter

Test: unit tests, no expected behavior change
Change-Id: I2cfd3488ed56e0e887d125b46b545dca26bee17f

[fidl] Requiring fidl library dependencies to be declared with using.

A `Library` may only use dependent libraries declared with `using name;` or
`using name as alias;`. During conversion from raw to flat AST, dependencies
are registered under their full qualified name, and alias if present, such
that access can later be more directly controlled. Also checking for duplicate
imports, and giving hints when encountering missing dependencies.

Some improvements around error messaging when encountering unexpected tokens.

Test: unit tested, fx full-build, on x86 and arm, also checking vendor

This reverts commit a252133fefe6dfc9984646e737e4a616222ff678. (It is the
revert of the revert, i.e. the initial change which is
https://fuchsia.googlesource.com/zircon/+/1100bd7e89caa04defae2b54cc833c729cb3c6db)

Change-Id: Ia5c0df9d9262d130bd933fa6d889e97d809b450b

[fidl] Revert "Requiring fidl library dependencies to be declared with using."

This reverts commit 1100bd7e89caa04defae2b54cc833c729cb3c6db.

Test: git revert
Change-Id: I0d679ef1d1c97e8bf29335177356e2018899fff9

[fidl] Requiring fidl library dependencies to be declared with using.

A `Library` may only use dependent libraries declared with `using name;` or
`using name as alias;`. During conversion from raw to flat AST, dependencies
are registered under their full qualified name, and alias if present, such
that access can later be more directly controlled. Also checking for duplicate
imports, and giving hints when encountering missing dependencies.

Some improvements around error messaging when encountering unexpected tokens.

Test: unit tested, and ran on many fidl files
Change-Id: I10c5ccfc2a4215240640507cf4a8d078f671d441